Earlier quoted context omitted.
Yearly reminder that a long time ago, chat services used XMPP and we were on the verge of having GChat interoperability with FB messages and I think Yahoo or something similar at the time. None of them really wanted to do it for business reasons, so they could “add value” (and charge for it)….same reason RSS has fallen out of favor (no good way to inject ads and tracking). IRC and Matrix still exist.
There is hope. The European Union's Digital Markets Act allows new messaging platforms to demand interoperability with the existing walled gardens. All it takes is for other jurisdictions to follow suit.
What will really happen is that there will be some subpar common denominator. An existing "walled garden" (WeChat?) would add support for this as well.
But this would wind up being rather insecure, because messaging services tend to use email addresses they don't control or phone numbers they don't control as identifiers. We'd have to wait for carriers and email providers to be regulated with the burden of solving this mess (for markets they aren't in).