This issue you are talking about is reliably solved ny owning your own domain. You can self-host it or use the commercial provider for it, either way you can change provider and retain your address.
And regarding these matrix guys, they are so overtaken by a NIH syndrome [1] that they couldn't even follow a common URI syntax. Are you really sure they are best fit to develop standard protocols of any kind?
There's very few projects that look at all interesting since the BitTorrent days, most everything is blockchain focused, and aiming to completely replace centralized at all costs with no ability to use central servers, even if it performs badly.
Owning your own domain has some disadvantages though. Especially with traditional federation. For one thing, your RasPi at home is probably not as reliable as Google cloud, and neither is a $5 month VPS.
Now you've got a single point of failure, that's on consumer hardware, with no professional IT staff managing it. I don't exactly want to pay for something that's less reliable than Gmail and WhatsApp, since an outage at the wrong time could hurt me way more than Google spying is likely to.
The only way I think self hosted makes sense for a typical consumer is if it can seamlessly transition to fallback servers, and you can talk offline on a LAN, and it doesn't rely on domains.
Like, I should be able to put a DHT record that says "I have linked accounts at these servers, try mine if you can, but if that fails here's a backup that we can meet at, that forwards to my main server".
That's what NIH syndrome does to people.
Tying identity to a server is insane in a decentralized environment. Decentralization is all about keeping Google sized companies from controlling things. But small companies come and go all the time, and nobody trusts them to be around in 10 years.
They aren't solving the SSL problem though. Mumble has a good solution their with their trust on first use connections. Self hosted really should ideally not need a domain name if you want individuals to host it. It should be something you can literally buy a $20 preconfigured hosting node, plug it in, and go.
Tox seems to be most promising of current ones, it just needs to fix it's mobile data use issues.