I really like the idea, but I prefer dedicated self-hosted services like Matrix. You can run a suspicious Matrix instance, but I think privacy falls away as you talk to any Gmail user via CoI.

"With XMPP and Matrix.org -based services you would still need to convince everyone to join your new network. Easy in theory, very complex in practice!"

I don't find that so problematic. People just needs to see a bit the inner workings to feel a bit comfortable and give it a chance.