Yes, because there wouldn’t be a centralized repository of posts.

yes, because being a user on a specific instance means relatively little

unless those largest instances defederate from the wider fednet it doesnt matter much that most users are on those instances, they still have the option to move to a different network, set up a community somewhere else etc

Yes and no.

Structurally, it'll remain decentralized, so one clear advantage here is that if the admins of a very large instance start trying something, my current understanding of how this works would let users ignore them. They don't control account creation, since any federated instance can see everything, so there's no meaningful way to actually block someone, they can make a new account and the rogue instance has no further powers to stop individuals. They could block whole instances that don't conform, but unlike Reddit, that doesn't get rid of them. Instead, it fractures the communities, which hurts everyone. In that case, a user protest wouldn't be a blackout like Reddit had, it would be a migration to another instance, and if other instances blocked them back, replacement communities would form.

Of course, this is a double-edged sword, it's harder for Lemmy to permanently end communities of hate and others that deserve permanent bans, as is always the case with decentralized authority, but that's the tradeoff.