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?
Tangentially related idea is how many sub-fediverse are their. Surly u could have 2 sets of instances that have all defederated the other group but are all federated with their own group.
We kinda had that on Lemmy until Hexbear decided to federate with Lemmy.World.
Also, I think Beehaw.org doesn’t federate with anyone, but it’s been awhile since I last checked