I assessed XMPP vs Matrix about 8 years ago, and strikingly, the basis on which it didn't make the cut still applies today. Here's what I responded to a sibling post: https://programming.dev/comment/5408356
In short, Matrix dug themselves into a complexity pit with an inadequate protocol, survived for a while on venture capital money (upscaling servers and marketing at all cost), all of it dried up, and now they are in financial trouble. Matrix won't disappear overnight, but is definitely losing the means to run the managed instances and the client/server ecosystem.