If they wall themselves off, that's not an inherently bad thing. One gigantic public forum was never the goal. One where anyone has the opportunity (but not right) to participate is the goal.

And if you don't like how an instance is doing things, you aren't "just stuck with it," like you would be with a centralized service. By way of example, my instance doesn't have downvotes, and I would be unhappy with one that has that capability turned on, like lemmy.world. If Reddthat decided to defederate and do Local Only, I would leave and go to another instance, but Lemmy would still be there.