That's a fair point, I don't have the same expectation of other os-level package managers.

There are alternatives in other projects that bridge build system and package management while directly addressing versioning of dependencies e.g. Spack, Conda, ... The first in particular does exactly what I want, so I suppose I should just be happy with it. Still would be nice for Nix to compete in the same space, esp. with the really slick translation from local to containerized environment displayed in here

Nix can do what you want, it just requires a little bit more manual intervention.

There's benefits, too, though - you know the C compilers, build flags, versions and libraries of everything that goes into making a Python library run. It's easy to change any of it, integrate with your own custom libraries or other languages and test the whole contraption in its entirety.

This is whats so powerful and wonderful about it, your end result isn't simply based on the direct dependents, but also includes the build chain for all things.

That's just not that common in other build tools.