While I think there are better services to be private and secure from a technical perspective, there's one killer security and privacy feature that Signal has that on one else does: usability. It's pretty hard to get my grandma onto Matrix, but it isn't hard to get her on Signal. The truth of the matter is that you can't have private and secure conversations if there is no one on the other side. So while I really do like Matrix and the like, I think of them as more alpha or beta type projects. I don't find that the bashing of Signal is helpful (like we also do with Firefox) because all it does is creates noise for people that don't understand the bashing is coming over a nuanced and biased point of view (we're mostly highly tech literate here on HN, it is a bubble. But people still read our comments that aren't). End of the day, if we aren't getting 1 click server installs (or literally everyone is a host), federated systems are going to become highly centralized at some point. PGP's always failed because the easiest way to hack a PGP email was to reply that you couldn't decrypt. It wasn't appropriate for the masses even when it wasn't difficult to use. Don't get me wrong, I love Matrix, but it's got a long way to go to get mass adaptation.
Fwiw, I remember a user awhile back offering a bounty for a decentralized pathway in Signal[0]. The idea was to create an AirDrop like system to help with things like local file sharing but then extend the project forward to create a mesh network. Seems like a reasonable idea to me. I think it may be more advantageous to try to push Signal in the right direction than rebuild from scratch. I'd highly encourage people with other opinions to participate in the Signal community because it is a crazy echo chamber in there and for some reason the devs treat it as a strong signal.