Sure, XMPP has problems, but ZERO of them comes from using XML. Thinking that using JSON will help solve the real problems any federated protocol would face is very naive and will lead to more than a few unpleasant discoveries down the road.
fwiw, nobody on the Matrix team (or that I know in the community) thinks that JSON is somehow fundamentally superior to XML - it’s just a data encoding. More recently we’ve been using CBOR over JSON for instance. Anyone who claims Matrix is about “using JSON rather than XML” to do the same thing is catastrophically missing the point.
