Off the top of my head. OMEMO running on good old XMPP. OpenPGP running on older but gooder SMTP, the only decentralized system that ever got really popular. Heck, OpenPGP running on XMPP for the mashup.
All with lots of implementations both server and client. All more or less ignored. Before suggesting we need new systems you pretty much have to figure out what isn't working with the ones we have ... and real stuff, not unlikely technical reasons that no one cares about ... and no, it isn't just bad user interfaces. People will overcome any user interface if they actually care enough.
OK, but this always happens. There are always problems with the old stuff (different problems for each) and all we have to do is overcome those problems and everything will work. But identifying what is wrong with something does not let us know what we should do going forward.
So what about the new thing: Matrix? What do we need to do there to make it so it does not fall into obscurity like everything else?
All with lots of implementations both server and client. All more or less ignored. Before suggesting we need new systems you pretty much have to figure out what isn't working with the ones we have ... and real stuff, not unlikely technical reasons that no one cares about ... and no, it isn't just bad user interfaces. People will overcome any user interface if they actually care enough.