In practice RCS is entirely run by Google outside China, the path to federation was killed around 2019 when Google decided every MNO should eventually move to Jibe if they wanted access to a global interconnected RCS, same on the client side.

I don't think it's too late as iOS finally supports RCS. But so far Google hasn't shown willingness to let unsanctioned clients connect to Jibe.

Except RCS doesn't work on anything except phones, while XMPP works perfectly fine everywhere. IM was vastly better on computers back when you could use applications like pidgin or kopete.

How do you use that on your devices that aren’t using GSM?

Cellular network is a terrible data transport solution. That's why we have TCP/IP on top of it.

I think that's the wrong direction, the cell providers will bargain for ways to put their junk into it.

XMPP installed on the handset by default should be fine.