All the federated structure in the world doesn't matter if you don't have users. I think we're better off encouraging bluesky to finish what they're building, mastodon won't ever get there.
I think the protocol is actually pretty open, and can be hooked into without them. I may be wrong though, I read into how it worked years ago.
In theory it is, but as of now you can't actually use any provider other than bluesky