It does appear to be an experiment in package management that got out of hand. Basic usability is lacking, as I was unable to even get a listing of available packages on a NixOS virtual machine due to nix-env requiring well over 1GB of RAM. This flagrant waste of resources doesn't inspire confidence in the quality of the tools.
I'd rather see existing packaging data leveraged rather than adding a new language to the mix. Something like a translator or interpreter for RPM .spec files, for example.
No data on this, but I'm pretty sure Nix is used mostly by for-profit companies trying to tame complexity on a limited budget.
It's not nice or pretty, but there's very little alternative unless you're ready to keep a whole team of developers just fixing Dockerfiles all day.
To each his own, it targets an entirely different niche.