The NixOS way of doing things (a layer on top of plain config files) is inevitable once configuration management is taken seriously: one needs to be able to merge configuration values from multiple source files; for example, a generic role plus some more machine-specific role, working together to produce one systemd unit or file in /etc. The modules in nixpkgs effectively make it possible to combine configuration values to produce most of the config files that Linux applications want.

If one weren't using NixOS to do it, it would be done with your own templates and configuration management, and probably without the huge advantage of being able to rebuild a whole immutable system without leftovers from prior configurations.