Email isn’t really an exception - it’s largely another federated system failure with most users on one centralized system (gmail).

You can’t even run an email server yourself without getting blacklisted these days. I doubt Twitter is going anywhere, I’m not even sure it can be replaced, it’s an unusual niche product of its time and network.

Mastodon doesn’t solve the underlying problems that lead to centralization either so most users end up on one server. Back to the same issues, except the experience is also worse than a competent centralized system.

To solve this for real you have to solve spam (need IDs), there can’t be a distinction between a user account and a server, and the server needs to be trivial to run and manage with automatic updates.