The thing with chat apps is that if everyone/most people I care about aren't going to use it, its useless for me.
I know nobody who uses XMPP, Session, or Simplex. I do know people who use signal and have convinced a few to use it. But that's it.
If nobody else, or very few people I know are using it, I'm not going to bother using it. This is a network effect type problem I believe.