My current Arch install doesn't have Python (because I don't need to), and I explicitly installed the same packages as in Nix.
This is just an example that some of their packaging policies are a bit odd, but they are working on this. Imagine using Nix on an embedded device. All those extra dependencies might make a difference in terms of resources used.
>My current Arch install doesn't have Python (because I don't need to),
That still doesn't explain why installing python is such a big deal for you, or why your desire to not install python is stronger than your desire to install mutt.
This is just an example that some of their packaging policies are a bit odd, but they are working on this. Imagine using Nix on an embedded device. All those extra dependencies might make a difference in terms of resources used.