Having a standardized system for that only makes Nix more complicated, not less. Now you have two parallel configuration systems to worry about. And the "simple" one will inevitably accrue more and more features to address its lack of flexibility. That's like the first law of config languages.
Now you inexplicably have two parallel configuration systems to do one thing
It exists for all software that has configuration and has a plugin system.
Think games. If I'm playing a game and I want to change the configuration to easy mode and up the resolution to wide screen I need to write a program or script to do it? No it should be a dumb setting configurable via gui.
If I want to modify a game beyond the dumb config or modify the config itself then scripts make sense. It's called mods. In games and in software with plugins, configs and scripts live side by side. It's one of the most common patterns in the world.
Additionally you're not seeing what I'm proposing here.
All configs and packages have to go through the "dumb" system. There is zero alternative. This is exactly what games do.
If you want to modify the configs then you go through the modding system. This can use the nix language.
Either way as it exists right now nix can't have a GUI for its config. If I make changes on a GUI how does the program know how to modify your config file which is a custom script that you could of wrote in a thousand different ways? It can't. You want a GUI, you need a dumb config, end of story. Unless you want nix stuck in the niche group of power users you need a GUI.
If thousands of users aren't following it, what's the point? Now the OS has to cater to the requirements of all those users.