> Plus, XMPP can be used for great amount of stuff non-IM related, while Matrix seems very focused around its IRC-like use case

I don't really agree here. While Riot is focused on a IM use of Matrix, Matrix itself is open enough to be usable in any use case requiring a payload to travel from A to B. You could think of Matrix use in IoT, social networks, forums, etc. Some people even made a working PoC of a blog using Matrix, and I heard some others are also working on building a system to bypass information censorship using Matrix as its only back-end.

I don't know XMPP enough to state which spec is wider than the other, but what I know is that Matrix isn't limited by what Riot or other Matrix-based IM clients can do.