I find it sustainable. I host my own instances. I can moderate as I seem fit.
Private forums have worked before. We have to relearn how not to have a corporate overlord without soul who moderates only based on law and advertiser friendliness.
But we also have to learn that we are not always welcome on other peoples instances and that's okay. People host these instances and they have opinions. Since it's the fediverse we can try to find a place where we fit in and there is a good chance we still get to talk to everyone else. Mastodon for example has a good way to move to another instance so it's okay to start on a big instance. I'm sure lemmy will get that too some day.
That's an undefined deal.