I take your point (it's the general point many use to argue for Snap/Flatpak/Nix/AppImage/0install/Homebrew/&c.) but if you want to ensure you're not replacing per-distro-package-manager-fragmentation with completely-arbitrary-chaotic-package-manager-fragmentation there needs to be some unity & consideration for what users want in order to ensure they'll willingly subscribe to your system of choice.

While I wouldn't call Flatpak "popular" with users per se, it's probably one of the least-worst alternatives that have come out. The horse Ubuntu has backed (Snap) may be the most used by virtue of being rammed down user's throats by projects with existing user capture (e.g. LetsEncrypt), but that's not going to make the debate go away: it just strengthen's the argument to return to distro packaging.