I see that fragmentation of runtimes is a problem. If all apps would simply use the same runtime, and a modern one, and there was a package manager that installs the missing dependencies, that would be nice.
The diskspace is a true problem too, just because of the fragmented runtimes.
But Distros are fragmented too. If simply everyone could unify, at least a bit, instead of at least 5 different big Distros competing, every app could just work. But thats not the case, so Flatpaks often work best, and maany packages are either only .deb, .rpm or even only on Arch