=== On NixOS, you do not need to be root to install software. In addition to the system-wide ‘profile’ (set of installed packages), all user have their own profile in which they can install packages. Nix allows multiple versions of a package to coexist, so different users can have different versions of the same package installed in their respective profiles. If two users install the same version of a package, only one copy will be built or downloaded, and Nix’s security model ensures that this is secure. Users cannot install setuid binaries. ===

Requirement to be administrator to install software is the root of all evils in managing OS.

As far as keeping everything in a special location (/nix or whatever), this reminds me SCO OpenServer 5: they had all the files somewhere in /var; /bin, /sbin and everything else were just symlinks. It did not work all that well.