Should/Would the Noble Sangha let go of the Dhamma?

It is not wise to fight decay & death. It is not wise to crave for what is not achievable. The Noble Dhamma says:

Enough friends! Do not grieve, do not lament! For has not the Blessed One declared that with all that is dear and beloved there must be change, separation and severance? Of that which is born, come into being, compounded and subject to decay, how can one say: 'May it not come to dissolution!'?"

DN 16

But it is difficult for undeveloped minds, full of self & lust for life instinct, undeveloped in emptiness (sunnata), to have such necessary wisdom. The Buddha his noble disciples have: "Disenchantment towards all of the world". It is very easy to type this saying of the Buddha but it is much more difficult to realise & embody it.

There is no rest in simply reflecting: "Beings are heirs of their kamma..."; because the very notion of "beings" is "self-view" and thus bondage to suffering. MN 117 says quite plainly that the doctrine of "kamma" is "defiled"; leading to "acquisitions"; and only siding in unliberated "merit". SN 12.51 says the Noble Disciple destroys "meritorious formations" via non-clinging.

The suttas clearly say the view of "beings" is the view of Mara or Satan. Mara is the Wicked One; the Evil One; the Destroyer of enlightenment. To quote:

Why now do you assume 'a being'? Mara, have you grasped a view? This is a heap of sheer constructions: Here no being is found.

Just as, with an assemblage of parts, The word 'chariot' is used, So, when the aggregates are present, There's the convention 'a being.'

It's only suffering that comes to be, Suffering that stands and falls away. Nothing but suffering comes to be, Nothing but suffering ceases.

SN 5.10

The Buddha did not teach Mahayana Compassion for all beings. The Buddha taught liberation from the world. Today, in the year of 2019, the world no longer has mainstream morality. Today, a bhikkhu, even an Arahant, is powerless to help the world. Talk of compassion is pointless. It is not wise to fight decay & death. It is not wise to crave for what is not achievable. The Noble Dhamma says:

Enough friends! Do not grieve, do not lament! For has not the Blessed One declared that with all that is dear and beloved there must be change, separation and severance? Of that which is born, come into being, compounded and subject to decay, how can one say: 'May it not come to dissolution!'?"

DN 16

Soon, the Totalitarian Asura will terrorise the world; like they are terrorizing many nations today. The only bhikkhus that will survive are those who do not offend the Asura, such as the LGBT promoters at Sutta Central. The bhikkhus teaching real Dhamma will end up like Jesus - crucified. The Buddha praised his Noble Bhikkhus who praised meeting an assassin.

These Sunaparanta people.. but if they take your life with a sharp knife…?

I will think, ‘There are disciples of the Blessed One who—horrified, humiliated, and disgusted by the body and by life—have sought for an assassin, but here I have met my assassin without searching for him.’ That’s what I will think….

Good, Punna, very good. Possessing such calm and self-control you are fit to dwell among the Sunaparantans. Now it is time to do as you see fit.

MN 145

Should/Would the Noble Sangha let go of the Dhamma?

When Nibbana (the far shore) has been attained, one can let go of the Dhamma, with reference to Buddha's Raft parable, MN 22: Alagaddupama Sutta: The Water-Snake Simile.

Should the Noble Sangha let them follow their inclinations which brings not only them long time suffering but for many? Resting simply in "Beings are heirs of their kamma..." or still share as much as compassion as possible to keep those able away from doing really grave wrong doings?

A being that has commited e.g. murder in a past life is not the same being that experiences the effects of that action later in that life or in future lives. The aggregates are constantly changing and renewing themselves until they are once and for all extinguished.

Therefore one should still help a person who is now suffering from the effects of unwholesome past actions. One should still practice compassion and wholesome intentions towards that being.

Firstly, you may need to choose your monastery or teacher carefully. Ajahn Brahm once said in a talk that he visited various monasteries and teachers and found that most of them were gloomy or stoic. He finally chose Ajahn Chah because Ajahn Chah and his disciples were happy and smiling, so they must be doing something right, he thought.

But if you listen to more of his talks, you will find him mentioning about abbots or senior monks who take the best food for themselves, and then mix the rest of the food together for the rest of the monks. So, there's no perfect situation with regards to the monastic order, I suppose.

Secondly, the last resort is to take the Buddha's Words (Buddhavacana) as your teacher and to take refuge in (and take example of) the ideal Sangha and not the conventional Sangha.

If you feel that you cannot trust some or all of the members of the Sangha, please use the Buddha's Words (Buddhavacana) in the suttas as your teacher, as it is said in the Mahaparinibbana Sutta:

Now, if it occurs to any of you — 'The teaching has lost its authority; we are without a Teacher' — do not view it in that way. Whatever Dhamma & Vinaya I have pointed out & formulated for you, that will be your Teacher when I am gone.

From Thanissaro Bhikkhu's essay on Refuge, we understand that we should differentiate the conventional Sangha from the ideal Sangha, and use the ideal Sangha as our example (since "not all members of the conventional Sangha are reliable models of behavior"):

The word Sangha, on the external level, has two senses: conventional and ideal. In its ideal sense, the Sangha consists of all people, lay or ordained, who have practiced the Dhamma to the point of gaining at least a glimpse of the Deathless. In a conventional sense, Sangha denotes the communities of ordained monks and nuns. The two meanings overlap but are not necessarily identical. Some members of the ideal Sangha are not ordained; some monks and nuns have yet to touch the Deathless. All those who take refuge in the Buddha, Dhamma, and Sangha become members of the Buddha's four-fold assembly (parisa) of followers: monks, nuns, male lay devotees, and female lay devotees. Although there's a widespread belief that all Buddhist followers are members of the Sangha, this is not the case. Only those who are ordained are members of the conventional Sangha; only those who have glimpsed the Deathless are members of the ideal Sangha. Nevertheless, any followers who don't belong to the Sangha in either sense of the word still count as genuine Buddhists in that they are members of the Buddha's parisa.

When taking refuge in the external Sangha, one takes refuge in both senses of the Sangha, but the two senses provide different levels of refuge. The conventional Sangha has helped keep the teaching alive for more than 2,500 years. Without them, we would never have learned what the Buddha taught. However, not all members of the conventional Sangha are reliable models of behavior. So when looking for guidance in the conduct of our lives, we must look to the living and recorded examples provided by the ideal Sangha. Without their example, we would not know (1) that Awakening is available to all, and not just to the Buddha; and (2) how Awakening expresses itself in real life.

On the internal level, the Buddha, Dhamma, and Sangha are the skillful qualities we develop in our own minds in imitation of our external models. For instance, the Buddha was a person of wisdom, purity, and compassion. When we develop wisdom, purity, and compassion in our own minds, they form our refuge on an internal level. The Buddha tasted Awakening by developing conviction, persistence, mindfulness, concentration, and discernment. When we develop these same qualities to the point of attaining Awakening too, that Awakening is our ultimate refuge. This is the point where the three aspects of the Triple Gem become one: beyond the reach of greed, anger, and delusion, and thus totally secure.

Also from the ATI Sangha page, it clarifies that the ideal Sangha is composed of those who are of the 8 types - those who are genuinely and sincerely trying to attain the four stages of enlightenment, and those that have successfully attained the four stages of enlightenment (stream entry, once-returner, never-returner and arahant):

In the suttas the word sangha (lit. "group, assembly") is usually used in one of two ways: it refers either to the community of ordained monks and nuns (bhikkhu-sangha and bhikkhuni-sangha) or to the community of "noble ones" (ariya-sangha) — persons who have attained at least stream-entry, the first stage of Awakening. The definition (ariya-sangha)

"The Sangha of the Blessed One's disciples who have practiced well... who have practiced straight-forwardly... who have practiced methodically... who have practiced masterfully — in other words, the four types [of noble disciples] when taken as pairs, the eight when taken as individual types — they are the Sangha of the Blessed One's disciples: worthy of gifts, worthy of hospitality, worthy of offerings, worthy of respect, the incomparable field of merit for the world."

— AN 11.12

Although "Dhamma" is in the title, the text of the question seems to be asking instead about the Discipline:

If the Sangha has become a group of householder wearing robes, should the Noble Sangha let go of their heritage? ... Would the Buddha allow monks to give up Sangha heritage in that case?

I read The Broken Buddha so it's not completely new to me to hear of someone saying something like, "the leaders of the Sasana are all but householders".

I wouldn't want to say that, and I couldn't say it about anyone (because I don't know them), but I suppose that what's written in the book has some truth in it, even if only that it's as-witnessed.

Similarly I read a little of the "austere Forest Tradition" -- specifically as contrasted with the living and so on in towns.

I don't have a specific answer for you -- I think there are several:

• You might want to find a monastery which you approve of -- a bikkhu, a group of bikkhus, who you approve of. People's biographies, monks and nuns too, often include their visiting and studying in several places, studying under several teachers.
• It's common to disrobe -- not that I'm recommending that, but perhaps that's what you meant by "give up Sangha heritage". Perhaps some people who do that give up parts of the the Discipline, but not the Dhamma.
• Perhaps consider being less critical of others -- I assume you are critical of others, "You shouldn't practice/live that way", "You don't have a proper teacher", "Your motives aren't proper", etc. To pick an example almost at random, there's this Wikipedia article about Ajahn Sumedho -- which describes him as "engaging and witty communication style", and pictures him and the monk who he's talking with laughing together. Then again the biography of Ajahn Chah says, "He sometimes initiated long and seemingly pointless work projects, in order to frustrate their attachment to tranquility" -- possibly you have some attachment to the way things ought to be, how other people ought to behave.

Is that, the degeneration, actually the reason why such as householder movements became that popular?

How popular is "that popular", what "householder movements"? Perhaps what you're talking about is an "observation bias" i.e. if you observe householder movements then that's what you see; and if you e.g. read the internet less then you wouldn't see them so much -- but I don't know what you're talking about. You see different things than I do, and perhaps you see them differently -- the all.

There are articles in the Appendix which say e.g. (in 1931) that ...

Already prominent laymen in Burma, Siam, Ceylon, and elsewhere, view with misgivings the present state of affairs and know that sooner or later some alterations will have to be made. Nearly everyone sees signs of decay in the Order, that Order that has continued for 2500 years, but today there are new conditions and forces in the world and unless something radical is done this decay will increase until either the Sangha dies out, or becomes a dead letter, the refuge of the ignorant and unworthy.

It isn't only monks of Western origin who say this. Anyway you might want to read The Broken Buddha assuming you haven't already. I think it talks about issues (i.e. problems within Sangha) which you question here, and quote various people who suggest solutions, and maybe you can find in it some people or organisations who you might concur with.

I don't know that I can answer this question, though, since I don't know what you mean by "householder movements".

I suppose that everyone (householders too) must practice on their own (as "islands"), AND that the Sangha is, has been, will be important.

Should the Noble Sangha let them follow their inclinations which brings not only them long time suffering but for many?

I don't know that it's possible to prevent people from doing what's harmful -- if you knew how to, perhaps you could help drug addicts? -- and if you can't prevent them then I don't see how it's a question of "letting them".

If someone only sees you as getting into arguments, and scolding people, etc., I suppose that's not effective.

Resting simply in "Beings are heirs of their kamma..." or still share as much as compassion as possible to keep those able away from doing really grave wrong doings?

I think the theory is that it isn't one OR the other (i.e. compassion OR equanimity) -- instead it's both (i.e. "and") -- or rather, all four (mudita and metta too).

Also you practice one whenever you can't practice another -- or perhaps one is an antidote to an excess of the other.