Re: "a Flatpak-only app store is the future"
It could be worse. It could be Snap.
I looked at Flatpak briefly. It requires that a Flatpak system be installed for the particular OS including the window manager. There was an version that seemed from the description to fit my case. It didn't.
As far as I can seen all these schemes seem to be a means of exchanging one lot of dependencies for another. Putting a directory with the application's dependencies in /opt is as effective as any but it's an old idea and, therefore, not shiny. Sigh.