How would that solve the issue of keeping repositories in a both well-tested and up-to-date state? It seems to me it would only make the problem worse because we'd have different, possibly incompatible or unmaintained PPAs
It would just let the OS engineers concentrate on the OS. Potentially making better OSes, maybe also more OSes to choose from, these being actually diverse with original ideas rather than making different app/version choices their key differences. "Just don't do what you can't really do well with reasonable ease, better just do the job which actually is yours".
The point of an OS is to facilitate the applications; there's an old law of project management that improvement happens mainly at the interfaces. The idea of making an OS that will run the same applications with the same interfaces but somehow be better is fundamentally incoherent.
