One thing I believe would at least help a little to increase adoption of alternative platforms would be to (and I know this is kind of problematic) learn a thing or two from the Windows 11 approach with the Teams integration.
The two biggest DEs for desktop Linux each have their own Matrix client (NeoChat for KDE and Fractal for GNOME), and I think it'd be a nice idea if they shipped them by default with the system and added some optional integration with the system. That said, I haven't used KDE in a while to be sure if they already do so tho, so my focus would be on GNOME. For instance, in the "online accounts" section there could be a Matrix login option that automatically logs your account in whenever you install another Matrix client, for example. Other nice to have would be to be able to have Fractal have its own native "appindicator" for users to be able to have it running in the background in a more seamless way (since GNOME doesn't support those out of the box).
XMPP would also be an ok option, but the focus here is on the UX/UI more than the privacy/usability of the protocol itself, and in both of these aspects the Matrix clients are way more polished than any XMPP alternative.
But yeah, any platform out of those is fine.