A relevant blogpost by Moxie about why the Signal app uses a centralized model (and thus doesn't allow other apps to connect to their servers): https://signal.org/blog/the-ecosystem-is-moving/
Still all code is open source and Signal's code does support federation. Moxie stated before that you can take the code and start a federated version of Signal if you want.
Re:Signal-desktop HTML tag injection advisory
by seba_dos1 on Tuesday May 15, 2018 @02:10PM
(#17076198)
Of course you can. That won't be Signal though, just an another network using its code, and there's already XMPP, so there's no need for that.
> (and thus doesn't allow other apps to connect to their servers)
"thus"? You can easily use centralized model and allow the client ecosystem to thrive. You don't have to do much, just don't actively prohibit them.
And I know the blog post, it isn't very convincing.
In Poland I don't see many people using Signal yet. I'm recommending Conversations to anyone who asks, which doesn't seem far away in terms of usability. On desktop it's a bit worse (I mean, I'm very comfy with my Psi, but wouldn't recommend it to a random person on the street), however Dino looks very promising and might fill that niche soon.