All fair points but completely irrelevant to most people who have either iPhones or Android phones and wouldn't have the first clue what an XMPP client is and aren't about to demand one.
Signal used with a username is entirely anonymous. Better, imo, to advocate for what is realistically achievable first then improve on it. When most are using WhatsApp, Gmail etc and are not persuaded of any adverse consequences suggesting unpopular ideal alternatives is futile.