What to do that wise would not disappear again?

Dhamma, as all religions (re-bindings) is open to use to bind one toward a certain aim. The Buddha Dhamma provides with the means of the robe to bind toward Undinding.

It's not possible to move on without firm bound, i.e. Nissaya, Upanissaya.

Only neither-sekha-nor-asekha (wordlings) or asekha (arahats) are without Nissaya. The fist instead to strong Nissaya toward sickness, oldage and death, bond to Samsara. A Sekha, one in training, has firm bonds to finish the path.

As devoted lay people but also monks in traditional countries say: "When ever you meet someone hard to meet (a Noble one, advanced monk), do what ever possible, that you don't loose him, even if he casts you hardly off, on and on."

Upasaka number-string pointed out some things, quoting commercial strategies. Those can be found in the Sangha-vatthu, the bonds of fellowship as well, and are ever since also misused for binding for lower purposes. Actually this commercial, directed toward gain, strategies are the main reasons why wise would certain not dwell under those tended toward the world. The highest four kinds of help go totally against the wishes of worldlings and therefore will not be desired in a community without even faith and plenty of ingratitude toward real help.

How ever, the question does lesser go into direction of community build or how one best helps but has the knowing "I am in need of help" as starting point while a certain community where one, according to ones tendencies dwells, should be always just for a time being.

There are many advices found in the Vinaya (starting by providing best dwelling, service, providing with specials...) and a lot of rules in regard of respect also point on the importand issue. People of a dhammic-community, desiring to increase the bounds toward liberation, should first of all always keep the thought: "May those more advanced in means required toward liberation, more liberated, soon come.", in mind.

If that point is clear, 1. that one would need the assistance of those more advanced, 2. a good desire for such is maintained, if that is the case, it would be worthy to elaborate more here. If not, than it would be wasted time.

Note: that this is not given for trade, exchange, stackes... for the world but liberation.

There is a good book about community building; Community Building on the Web : Secret Strategies for Successful Online Communities – by Amy Jo Kim.

The most knowledgeable people are those who are able to answer almost any questions. They are extremely valuable but are also quite likely to retire from a community in part because they don't need the community to learn and their lack of peers is proportional to their expertise.

If one is looking to keep the knowledgeable people around one should protect them. They do need to be protected from harassment and it is the job of the moderators. The moderators have to work with them and this can be difficult.

Other than that they can also just lose interest in teaching or become otherwise disinterested.

Related Sutta stories;

There is a story about monks quarreling and the Tathagata leaving the community because the monks did not settle their quarrel.

There is another story about The Buddha not teaching in an "impure" assembly which results in a monk being removed by force from the assembly.

Another is when Ven. Sariputta stops teaching because a monk harasses him by openly disagreeing in front of other monks and nobody defends Ven. Sariputta. Later Buddha admonishes the monk for harassing Ven. Sariputta and admonishes the community of monks for not defending the Elder.