> who wants to run debian stable or centos as his main OS?
I haven't used these two in particular, but if they support flatpak, why not? You'd have the same packages as on the server plus the latest versions of GUI apps.
I've used flatpak on an Ubuntu 16.04 base without issues. There are limitations, for example you can't easily replace/update your window manager through flatpak or snap. But for user-facing apps, it's really a great step forward.