I really wish Matrix had been more successful, but it has some pretty core problems that prevented it from gaining more traction.
It fell into the same trap as XMPP, though perhaps even worse, with a focus more on its protocol and specification than a single unified product vision. The reference server implementation is slow, and using a language not optimal for its purpose, with alternative server implementations left incomplete and unsupported. It took a long time for them to figure out voice and video and for it to work well, and the “user flow” still isn’t at Discord levels.
I’ve rooted for Matrix for a long time, but as a former XMPP evangelist, to me the writing on the wall says it isn’t suited for success either. I’d love to be wrong, but I don’t see a way through.
I do think the other home server implementations gaining parity (production-ready) with the reference home server would go a long way. I haven’t run a home server but I’ve heard from those that have that it really has a hard time scaling. (Though this serves as impetus to give it a try over spring break)
Which brings me to the caveats of the protocol, I personally don’t think the design is ideal, it’s more described as a distributed message bus, what I’ve read of the spec it’s over engineered, it made good decisions wrt using modern web technologies (JSON, WebRTC), but it didn’t scope itself to the particular task.
That said, I haven’t written a federated protocol, and they have. But if I was going to, I’d really want to look at Discord and see how to copy a lot of that model, but break parts of it out to facilitate federation:
I originally wrote a huge hypothetical design here that I speculated would fare better, but honestly the specifics become less relevant, point is that the shared state of rooms is a real challenge, and one out of scope for just a federated instant messaging system, and I’m no longer certain it’s viable.