I think I should have been clear. It does not make Unix single user machine but it is more useful for cases where a single user is the main user of a machine like the laptop user.
My reasoning was very simple. Nixos makes it easy to install packages on a per user basis. This would mean there is a lot of redundancy if another user also needs the same package. A snapshot/dedup filesystem will easily solve the problem.
Multi-user Nix systems use a "nix daemon", which user's commands send requests to ("please build this version of Firefox with this version of GCC, etc."); all of the results go in the main "nix store" (usually /nix/store).
Nix doesn't store duplicates; a hash is calculated, based on the inputs (source code, compilers, libraries, etc.) and if an output with that hash already exists, it will be used. If not, the configured binary caches will be queried, to see if a pre-built binary can be downloaded. If not, the inputs are fetched (following the same process) and the build is performed.
When a user installs a package, it goes into the system-global package store, and the user's symlinks are updated. When a second user installs the same package, they just get a symlink to the original. There isn't any duplication.
My reasoning was very simple. Nixos makes it easy to install packages on a per user basis. This would mean there is a lot of redundancy if another user also needs the same package. A snapshot/dedup filesystem will easily solve the problem.