Earlier quoted context omitted.
Nix feels to me like a cool but somewhat hacky v1 of a really great idea. I'm hoping someone creates a much more elegant successor. If Nix is CVS or even Subversion, I'm hoping for Git.
The neat thing about the functional design of Nix is that you don't have to reinvent the wheel: just make better steering. Everything about Nix can be trivially refactored. Every part is neatly isolated from the rest.
Tweaking a few NixOS modules it replacing a package is easy, but neither the Nix codebase nor Nixpkgs is characterized by perfect modularity or uniform layering.