Do they get adopted by other instances? Are they still accessible from other instances? Can you still post on them from another instance?
Edit: From my understanding every instance that deals with a community has a cached copy. Will that copy disappear after a certain time, because it can’t phone to home anymore?
This.
If we don’t want things run so as to squeeze every ounce of value out of users, then things can’t be run by profit-seeking entities. They basically need to be run by altruistic people doing it for pro-social reasons, or we stay on the enshitification train.
If people are really invested in a community remaining a forever space on the internet, we just need to develop the features for migrating those communities to new instances, and then having that community shell out for their own host.
That is where stability will come from. Communities self-hosting.