That's the advantage of Nix's top-down approach. You declare everything on as high level as you want/can/wish. You specify the package plus your customization for it.
It's not that much different than a one-liner that installs the package plus uses echo to write your config file. The magic is the "forced" reproducibility by default.
Conceptually, I think nix is great. Most of us can appreciate the value of a perfectly reproducible system, etc.
That said, unfortunately nix is a practical option for about zero percent of commercial shops. Sigh.
I did actually work at a place that tried it once. Summarizing, the introduction failed through some combination of politics and the perception that nix would introduce a lot of complexity vs a standard vendor distro.
It's not that much different than a one-liner that installs the package plus uses echo to write your config file. The magic is the "forced" reproducibility by default.