If Lemmy, or any other fediverse social network, ever got that big, you would be guaranteed there would be plenty of splinters of de-federation in the network. There would be small networks totally isolated from the mainstream cluster, and others that only federate parts of it. You could choose to hang out in some counter-cultural bubble, or choose a curated connection to the biggest networks, if you didn’t want to engage in them fully. The trick would be finding “your people,” but the tech works.