For GNU/Linux, Flatpak it might be if GNU promoted a trully FOSS repo; but they already made their own Flatpak with GUIX, and the inmutable distro it's GuixSD itself with Plasma.

On 'the ship already sailed', Gnome's performance it's really bad compared to Plasma.

GNU would never promote Flatpak, because Flathub isn't FOSS-only, and bringing proprietary software vendors into the Linux ecosystem seems like it was one of the goals of Flatpak into the first place.