It depends on how many communities are on Meta's instances. Imagine if a year from now, Meta's instances are all the rage. It's like lemmy.world on crack, where 95% of the communities you see browsing all are from meta.world. Everybody hates that it ended up this way, but that's just the way it shook out because Meta has the best performing servers and a huge population of shitposters on Threads creating funny memes who probably don't even know instances besides meta.world even exist.

But then Meta decides they're done with the fediverse and decides to close off access to meta.world. Now 95% of the fediverse is effectively vanished, and you either have to switch to Threads to regain access to those communities, or you have to stay on the 5% of the fediverse that's still left and basically start from scratch all over again.