This isn't necessarily an endorsement of one protocol/ecosystem over the other, nor do I have direct experience with integrating Matrix or XMPP (though I run the latter on my home-server for family), but XMPP has seen a few large deployments, including in healthcare (in the UK)[0][1] and in Germany[2].
The consumer-facing client ecosystem for XMPP has indeed seen less rapid development than Matrix (the latter probably benefits from a more cohesive approach), but the server ecosystem for XMPP is very mature, and servers such as Ejabberd are known to scale to hundreds of thousands of connections on a single, modest host[3]. Obviously, that's only one part of the puzzle, hence why Matrix was chosen here.
Still, it'd be interesting to see how the two evolve and compare down the line.