> Simple package based distros like Arch basically just extract archives. Very few packages trigger post-install steps which usually just (re)generate something like initrd.

Sure it's simple, just like 80% solving the problem is usually significantly simpler than solving it 100%. Nix (and its generation) is the only package manager that actually works.

Try installing kde and gnome, then uninstalling both and check how many packages remain.

Nix can do that with the whole world with nothing residing.