The thing about Nix is that it's the first project, as far as I know, that tries to solve the problem of package management with a fundamentally different approach. Most of the solutions to date were just okay: they worked with a tolerable degree of failure.

A solution that works good enough is often a serious obstacle for developing a solution that works really well. E.g. Unix works just good enough to preclude the adoption of a new paradigm in OS'es such as Plan 9.

Treating packages like immutable values sounds like a genuinely new idea. So what I think is that people don't go "this seems better than my current development toolchain", instead they think that the idea that underlies Nix might result in disproportionately more reliable tools in package management. That's worth fussing about.

Disclaimer: I am not associated with Nix in any way.