Wouldn't say there's a steep learning curve for the language itself, it's pretty easy to get a grasp around it imo. Here's a helpful page I used to quickly get familiar with the language: https://github.com/tazjin/nix-1p

What's rather messy about Nix is nixpkgs with its helper functions all over the place alongside pretty shallow / non-existent documentation (which is unrelated to the language). Thankfully they've started to work on that recently: https://discourse.nixos.org/t/documentation-team-flattening-...

I think it's all too easy to say they should have just used an existing language. The most obvious feature of the nix language is that it's lazy, because they want their enormous collection of configuration dictionaries computed on demand. There are no mainstream languages that are lazy.

Well, there's Haskell, and I expect any Haskell programmer to be able to pick up Nix very quickly.

Guix has gone with a strict language, Scheme, but I understand they have their own monadic DSL to cope with the peculiarities of following the Nix model. So again, not something that a mainstream language can do well, and even as a Schemer, you're going to have to learn their macro language.