The reason it is sort of complex is simply the fact that the problem domain is complex. I don’t think there is a big deal of accidental complexity around nix, it is mostly just essential, which can’t be reduced.

The same is true of the often bashed systemd - system boot is a hard problem, it gives a correct solution over incomplete but simple ones, and thus it will be more complex. But some complexity is inherent, so we just have to deal with that. (Rust could be another example - correct manual memory management necessities some added complexity )