All these years later Matrix only has... The ambition to some day try to offer the core privacy features Signal already delivered back then. Some of the most basic stuff is, you believe, almost kinda sorta done.
This is, to be clear, much better than just sitting back insisting you were right but not lifting a finger. But for an actual user who needed privacy and security any time between then and now - and for future users who need it between now and whenever you get this stuff working in the real world, it was Signal that delivered. Moxie was right so far.
And you know another thing? Email works great to me, and it's decentralized. I have my email server, with my domain, so I don't depend on anyone to provide me a service. I have full control of my data that is on my server. I can even send encrypted emails with GnuPG without any problem, and it's as secure as Signal, if not better. I can use whatever client I want, a fancy application, a web interface, or as I do a small CLI program since I don't use graphical user interfaces.
Sure, having a chat protocol that is decentralized like email would be great! I wish that Matrix will evolve in something usable in the following years, we need that. I need a chat application where I can have a client that I can use in my terminal, and Signal doesn't do that (Telegram does, for example, since the API is more open, and it's what I use second to email).
Have you looked at signald or Signal-CLI?
https://gitlab.com/thefinn93/signald https://github.com/AsamK/signal-cli