alt Hacker NewsNix already has a ton written in it and it works... Magically.
Gradual typing for a system like this is the proper thing to do. You can always write a static linter for sections you need tested and maybe eventually you'll be able to type nixpkgs as well.
Any solution to the nix typing problem NEEDS graduality. Nixpkgs is too large to make it possible to have one commit that fixes everything. Starting from scratch is not a realistic option either.
Maybe you need graduality, sure. Plenty of systems (e.g. Typescript) have added a real type system that works gradually onto an existing system. While there are compromises to that approach, it's still a lot more effective than contracts IME.