I tried to migrate friends and family to Signal a couple of years back. Only managed with 3 people :/.
Unfortunately here (Italy), WhatsApp is too much widespread as reach out method now. Especially for businesses, from restaurants to, say, the family doctor.
Probably he means that Debian stable is outdated by definition (e.g., Debian 12 it just released to stable, and has Gnome 43 instead of 44), and to stay on top of new versions you would eventually rely on flatpak.
However this is a false problem, because if you want a updated version, you just change your update channel from ‘stable’ to ‘testing’ in the sources.list conf file etc voilà you have a rolling release with fresh stuff.
The majority of other distros value package managers that allow for complex graph evaluation of dependencies, and the ability to roll back. This is granted with rpm and Deb, but not for pkgsource, which is a pretty lightweight format compared to those.
As for AUR, the major distros (Ubuntu, Debian, Fedora) support 3p repositories as well. The main concern is security. IIRC one of major complaints for AUR in the past was that it didn’t foresee a strongly secure distribution system.