Well universal chat (like universal e-mail) is either going to be a common open protocol (does not seem very likely given Apple and all the other players) or is going to be something like this on the client side. Although its a lot of work, it does seem more possible. The only pity is it can't solve connecting to services that I don't use like Facebook, Instagram, Whatsapp.
The EU is forcing the big chat companies to open their gates. They have until April of next year to comply, so we might see a common protocol for chat pretty soon
They need to be interoperable. For example if you're on Signal but want to chat with your parents who are on FB Messenger, then you would be able to chat to them via Signal.
You can envision each chat apps as different instances and these communicate to eachother with a common protocol, just like ActivityPub in Lemmy.
That is really going to be interesting, yes! It is seriously needed despite what Apple will say. And if implemented correctly it can still be E2EE but with our own client apps.