Agreed that Matrix isn't a "modern alternative" to XMPP, but for different reasons. At its core, Matrix is a decentralised object database for conversation history (like NNTP). XMPP is a message passing protocol (like SMTP, but extensible).
Matrix is not "all pull-based" - that's just the baseline implementation. Folks have already implemented push-based transports for Matrix - e.g. COAP or WS.
edit: [Disclaimer: i work on Matrix]
