My biggest problem with messaging applications as a whole is that I need so many of them. I have a few contacts who are reachable with Signal, a few more who are reachable on Hangouts or whatever Google is calling its latest thing, and a few more who are reachable with Skype. That's three programs, to communicate with three different subsets of people. I'm sure if I used Facebook Messenger and Whatsapp, I'd have two…
I'm not sure if I want that. Most of my communication over email can be read by Google, regardless of myself using Google. Moreover, there's the risk of things like AMP for email unilaterally changing the protocol in ways that as a user I might not like, but that I can't really influence through my choice of client.
I don't believe matrix.org will become such a thing, but if Matrix becomes as successful as email, I fear it's practically inevitable for a Google, Facebook or whatever to arise and become the dominant player, whereas that is practically impossible with, say, Signal.
I do see the value of the world that's made possible by technologies like Matrix, but I'm not so sure if they're possible with social processes evolving the way they do. I feel like Signal may be the best we can get.
(Though I'm certainly happy that Matrix is trying to prove that wrong.)