I’d say flatpak isn’t the future because it’s already here and seems to be universally accepted as the cross-distro package manager.
I do like how the Nix package manager handles dependencies, but it’s not suitable for app developers packaging their own apps because of its complexity.
If a better flatpak comes around I’d use it too, but at least for graphical apps I don’t know what it’d have to do to be better. In my opinion, flatpak is a prime example of good enough, but not perfect and I’d be surprised if there was a different tool with the same momentum in 15 years (except snap, but they seem too Ubuntu specific).