Because empirically Matrix is what governments appear to be using - e.g. https://tchap.beta.gouv.fr/ for France; https://messenger.bwi.de/bundesmessenger for Germany; Delta for Ukraine; equivalent for UK & US, etc.
Matrix also has a completely different featureset to XMPP or IRC: E2EE by default; conversation history as a first class citizen; HTTPS+JSON APIs by default; E2EE Group VoIP (almost) integrated; etc.
> Software developers earn way more than most public sector workers - is this justifiable?
This feels like a false dichotomy. The implication is that public sector software developers are paid less than average private sector software developers. This may well be the case in some places - but the answer there is SURELY to pay public sector software developers the average going rate, rather than conclude that funding private sector FOSS maintenance is not justifiable(!)