The problem is that people don't want to change, because it takes some effort. Why would people use WhatsApp instead of Signal otherwise?
A huge part of this is fonts. Users prefer proprietary fonts, and when you open files using them in the alternatives it tends to look like terrible. You will not convince users that this is on them to fix, and to be honest they're right, it's not their problem.
Just look to the federal United States government using it for communicating military strikes, and including journalists.
At least we don't seem to have things which are close by UX and security at the same time.
Simplex is fine, but still feels a bit raw.
Everything else is either untrustworthy because of the closed code or no e2e encryption or custom encryption schemes (WhatsApp, Telegram, any Asian messenger) or unusable from UX perspective (Tox, Matrix).
If I talk to someone on Signal today, I know that they are probably using the official Signal on the other side. With the guarantees that I know from Signal. Now what if half of the users of Signal were using a third-party app? How much can I trust this app?
Say Matrix has a bridge to Signal. I talk to someone over what looks like Signal from my end, but it goes to some third-party server that pretends to be Signal and then relays those messages to my friend on their Matrix client. As a Signal user, I cannot know it, but my conversation is not E2EE anymore. And it kind of defeats the point of using Signal entirely, doesn't it?
I guess my point is that in terms of security, there is value in making it possible to verify that both ends are using the official Signal app, by locking it as much as possible (e.g. with DRM-like technology). But of course it's annoying to be locked in. Even though I don't feel personally super locked into Signal: I could move to another similar app in a minute. But again people tend to be lazy and don't want to switch apps. It's a hard problem, I guess.
We've been working on our next major release for a long time now to better support modern protocols. But as an unfunded Open Source project it's hard to get things done quickly when it's a "free time" only project.
Perhaps we'll see a return of apps like Pidgin soon.
