That ship has sailed

Not if we don't want to.

To catch on, the next solution must be better than the last.

It’s too cumbersome for average folks.

one XMPP server gets big and usernames get squatted and dealt with at the discretion of the server admin.

Only solution is something like how Farcaster does it where you at least get to keep your public key the same and usernames are optional.

https://github.com/farcasterxyz/protocol#3-identity

Or go the urbit route and make the addresses totally user owned.

Federate messaging servers and make addresses domain specific. Works fine for email.

People lose their gmail accounts all the time for weird reasons. Maybe you are specifically talking about a world where everyone self hosts their own servers?

Gmail sucks, but not for any reason caused by email the protocol.

It is the fault of email, the protocol, yes.

If email was better designed, it would not allow these situations to happen. For example, if server operators didn't own your account, there would be no problem.

It is a protocol issue.