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

Sounds like a package manager or package problem. Competent package managers (e.g. dnf) remove unneeded packages and all their owned files. Albeit, I think with apt you need to do a manual autoremove to remove orphaned packages.

Not suggesting that they're equivalently powerful compared to nix, but this specific thing shouldn't be a problem with traditional package managers.

A common issue with most package managers is that if you have A installed, and then you install B which depends on C, and that C happens to also be an optional dependency of A, then uninstalling B will not uninstall C as C won't be orphaned (because of A).