Fortunately, "just use Nix" doesn't do much on its own. People usually want GCC or another complete C toolchain, a C standard library, etc. and this implies tha

Fortunately, "just use Nix" doesn't do much on its own. People usually want GCC or another complete C toolchain, a C standard library, etc. and this implies that they will use nixpkgs or one of its forks. If people try to "just use Nix" in anger, then they will almost certainly be funneled into using nixpkgs as a matter of practice. The main problem with reproducibility in science is that most scientists are not actually interested in doing science. Of course software will not fix this problem.