Clearly you haven't dealt with a Mastodon instance having a major defederation event.

For most users, regardless of the validity of the defed, the user experience is terrible. Their social graph just suddenly, stops working, the people they follow can no longer see their posts, all because of the actions of a few bad actors or administration failures.

This paired with the fact that maybe only Firefish or Misskey lets you (mostly) seamlessly migrate to a new instance with your data intact, and the lack of a standard way to see what followers you will actually keep when you migrate, means that the defederation experience is sucks, and migrating to a different instance to escape that is a pain in the ass.

Meta has already shown it has piss poor moderation in the best of times, and actively boosts incendiary content in the worst of times, all while collecting, profiling, and exploiting your data. It's literally inevitable that they're going to break the rules of all but the free-est of free speech Instances, so for the privacy, safety, and headaches of everyone in the fediverse, we might as well save ourselves the trouble.**