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.
You can also run your own mail servers, even non-technical people could do that with for example https://thehelm.com/. Haven't tried it out myself, though.
"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.