I think even pre-LLMs, the problem was more the size of a community rather than whether it was closed. The bigger it grows, the more it orients itself towards the lowest common denominator. Every community I ever cared about was rather small. Places like Hacker News, which I think is among the better large ones, I honestly don't think I'd care much if it disappeared tomorrow.

But now with AI unfortunately I agree. Not sure if there is a way around closing off communities. Even for communities that actively use AI within.