It doesn’t clash at all. If startups keep re-inventing the wheel just to have shiny things to sell investors on we end up with fragmentation which is terrible for interoperability. For example it’s impossible to send an encrypted message to a Matrix user using any XMPP client, since Matrix bridges can’t handle end-to-end encryption. Why? Because the company behind Matrix just had to cook up their own protocol instead of building on (and thus improving) existing internet standards. This is bad for interoperability and privacy.

You also seem to have trouble understanding that there can be multiple factors at play, not just a single one. I’m not arguing just privacy or just interoperability, but a combination. XMPP performs well in both while Signal performs slightly better in the first one while completely failing the second one.