I think this is a prime example of why the fediverse won't even come close to replacing Twitter for Twitter users.
The same thing has happened in the free software movement in general; some folks have called for copyleft-only, and the rest of the world has largely ignored them and is fine with shipping BSD licensed software along with GPL licensed software.
In reality biggest servers still federate with most servers. With latest changes in Mastodon you can now see whom they don't federate. It's not huge list, and if you browse those who don't federate with, it's pretty obvious why.
A lot of those defederated services advertises themselves as some sort of free-speech absolutist alternatives.