But you're confusing what sides of the developer/user aisle you're on.

Yes, there are probably a better distribution for the user to use if they want to save disk space. But Flatpak is not a distribution, it's a delivery mechanism. Are you really gonna tell people "Hey, don't use $YOUR-CURRENT-DIST, use Gentoo instead if you want to use my application" if your goal is to provide value for as many users as possible?

It's simply not possible to ask people to change their distribution because you as a developer prefers a different one. Flatpak is for all distributions, and doesn't require someone to change from what they know and use already, it'll work for everyone equally.

it’s been tested and almost all library packages are only used by one other package on the system. And almost all of the sharing comes down to a very limited set of libraries.

Flatpak solves this with platforms. You can base a package on the gnome platform for example and get a whole bunch of the most used libraries.

But even without platforms, disk space is one of the most abundant resources right now. You could install millions of libraries in the space of a single AAA game.

Ex-(long time)-Gentoo user here. Linux packaging is insane without resorting to something like this. The problem could be fixed but it would require coordinating long-term-stable (like: 5 years, minimum) versions of an entire, fairly large set of basic libraries, across (at least) every distro that matters. For desktops this would probably have to extend to long-term-stable versions of X/Wayland and their related libs, plus QT and GTK.

Is that bad because you don't have the space to store it or is it bad because you don't like it in principle?

If its the former then its a non issue. Storage is cheap and plentiful and code takes up little space relative to other assets such as multimedia content.

If its the latter then I don't know what to tell you other than practical issues are more important than style ones when making practical software. The community has voted and decided that classic package management doesn't cut it and easy of installing software is important.

And I have to agree with them. Time and time again we've been shown that friction to users doing whatever it is they want is the biggest barrier to adoption. Its easy for tech types like ourselves to get stuck in the weeds of the computer. But to most its a means to an end and if it isn't facilitating that then they will find another.

Just use a deduplicating filesystem. Both btrfs and xfs can deduplicate identical files.

Snaps can do this