> Using shared libraries is optimizing for a very different set of constraints than nixos, which iirc keeps like 90 versions of the same thing around just so everyone can have the one they want.

This isn't really true. One version of nixpkgs (i.e. a specific commit of https://github.com/NixOS/nixpkgs) generally has one version of every package and other packages from the same nixpkgs version depending on it will use the same one as a dependency. Sometimes there are multiple versions (different major versions, different compile time options, etc.) but that is the same with other distros as well.

In that sense, NixOS is very similar to a more traditional distribution, just that NixOS' functional package management better encapsulates the process of making changes to its package repository compared to the ad-hoc nature of a mutable set of binary packages like traditional distros and makes it possible to see and rebuild the dependency graph at every point in time while a more traditional distro doesn't give you e.g. the option to pretend that it's 10 days or months ago.

You only really get multiple versions of the same packages if you start mixing different nixpkgs revisions, which is really only a good idea in edge cases. Old ones are also kept around for rollbacks, but those can be garbage collected.