I’ve begun to wonder about this question. There’s nothing implied in the existence of the fediverse that it will include every site, and there’s nothing implied in the existence of every non-fediverse site which would stop them from forming their own fediverse. One could technically say email providers have already achieved this, though this only really applies to email providers which doesn’t make it as useful as the fediverse we know and love. Are there actually other fediverses out there?
Before the fediverse we already had the IndieWeb with Webmentions and microformats. And before that we had the Tent protocol. And before that we had the blogosphere with webrings, Pingbacks, Trackbacks and RSS.