Nix is not something exclusive to NixOS, and people are already using it to make reproducible configs that work on more than one OS.
I'm even using Ansible in what I'm currently building with Nix, because it does one thing well that I need to do: distribute files and run commands on a lot of hosts at once.
In my head they're different use cases. Nix is amazing for a living build. Ansible is more pigeon-holed to production systems where you don't want (or need) that history baked into every system
That is, until a new Ansible version breaks playbooks again, or an OS is updated in a way that messes with you playbooks, or a package is removed from the playbook but not the installed system...
Ansible is good for ephemeral containers or VMs, but any more permanent system will eventually deviate from the set configuration.