a reason for long-lasting defilements

According to AN 6.87, there is actually a list of characteristics, and such a person will never be able to have the right mental state or moral capacity to learn the Dhamma:

"Endowed with these six qualities, a person is incapable of alighting on the lawfulness, the rightness of skillful mental qualities even when listening to the true Dhamma. Which six?

"He has killed his mother; he has killed his father; he has killed an arahant; he has, with corrupt intent, caused the blood of a Tathagata to flow; he has caused a split in the Sangha; or he is a person of dull discernment, slow & dull-witted.

"Endowed with these six qualities, a person is capable of alighting on the lawfulness, the rightness of skillful mental qualities even while listening to the true Dhamma. Which six?

"He has not killed his mother; he has not killed his father; he has not killed an arahant; he has not, with corrupt intent, caused the blood of a Tathagata to flow; he has not caused a split in the Sangha; and he is a discerning person, not slow or dull-witted.
AN 6.87

According to MN 86, Angulimala killed many human beings, but he could still change and become an arahant. So, probably, he did not commit the transgressions above (killing father, killing mother, killing arahant, injuring Buddha).

So in short, if one did not commit the acts mentioned above or have those characteristics, one could theoretically completely turn around and become an arahant in this very lifetime.

From Dhammapada:

85. Few among men are those who cross to the farther shore. The rest, the bulk of men, only run up and down the hither bank.

86. But those who act according to the perfectly taught Dhamma will cross the realm of Death, so difficult to cross.

87-88. Abandoning the dark way, let the wise man cultivate the bright path. Having gone from home to homelessness, let him yearn for that delight in detachment, so difficult to enjoy. Giving up sensual pleasures, with no attachment, let the wise man cleanse himself of defilements of the mind.

89. Those whose minds have reached full excellence in the factors of enlightenment, who, having renounced acquisitiveness, rejoice in not clinging to things — rid of cankers, glowing with wisdom, they have attained Nibbana in this very life.

Dhammapada

In real life I've found that how and with whom you live has long-lasting effect -- and that finally changing that gives "a chance to become undefiled", and then, "the opportunity to give effort arises".

That's analogous to what you were saying, about living in and eventually leaving hell.

So, in real life, meeting regularly with, being influenced or tutored by people who are better, saner, more together, kinder, wiser, more disciplined -- that change of environment gives you a chance to become like a different person.

I think this observation is in line with Buddhist doctrine, about finding good friends.

The kind of change I'm thinking of includes (but is not limited to only) letting go of addictions, substance abuse. I suspect the hell might need to be replaced with something else -- a different experience, influence, environment, role model.

Also I'm not sure "weak-minded" is the right diagnosis. Instead the "root poison" according to Buddhism is ignorance. So it may be fixed by someone who has better insight into the condition.

For example if a person were sick and a doctor knows how to cure that illness, I wouldn't call that "weakness" of the patient -- it was that they needed the doctor's knowledge, view, know-how, and relationship.

That's an example, I'm not saying that doctors are the only people with any insight.

Not associating with wise, association with fools, not meeting the wise, not tending toward them, not lending an ear, holding on corrupt stand, is the only outwardly reason for long lasting wandering on.