Nix, the package manager, can be installed on any Linux distribution or on macOs. It's not confined to NixOS. It runs pretty much anywhere Docker does.
Will it? Nix solves a unixy problem in a unixy way. The Nix/WSL story might get happier at some point to solve a problem for Windows based developers of Linux based servers, but that's effectively just "running nix in docker", except that the interface is WSL instead of Docker.
I know some people write Windows software from Nix, but this is a way for Linux developers to make desktop apps for Windows users, not a solution to any problem Windows developers have.
I can see three broad reasons people develop on Windows:
1. They want to create a Game or Windows end user UI App (e.g. Overwatch or Photoshop)
2. They write some internal corp tooling or B2B software for windows shops (e.g. gluing some SAP or Oracle garbage together)
3. They want to write a server application (which means deploying on linux, which is basically the only server OS left). But they do not want to run linux as their desktop OS for corp or private reasons.
The 1st category probably has limited use for either docker or nix (in the absence of first class windows support). Might still be useful for tooling though.
The second category probably has use for docker mostly as a shitty linker, maybe also for isolation/security (I don't know the windows docker ecosystem).
The 3rd category can and totally should be using nix and I'd guess is at least double digit market share (so not insignificant; e.g. before Google banned them, a large fraction of Googlers had windows notebooks).
Yes. If you're trying to develop Linux software from Windows, Nix works to the extent that the rest of your software works. But the experience of working on WSL is not the experience of working on Linux. And it isn't likely to help you build your Windows based programs.
