For a long-running program, like a network service or whatever, yes absolutely runtime assertions are a poor substitute.

For an impure program, where which branches of code are hit or aren't hit depends on the result of network calls, filesystem state, user input, etc, then yes, a type-system will be more powerful.

However, nix is more-or-less pure. By fully evaluating the output, you can hit all of the non-dead code, and hit every runtime assertion every time.

For a completely pure program, runtime and compiletime might as well be the same, running it and caching the output is the exact same as compiling it, and if it fails to run and outputs an error, that's the same as it failing to compile.

I know the "more-or-less pure" is doing a lot of work there, and in reality there's a lot of bits that aren't really pure, but it's close enough that a runtime type system gives you a lot more than you'd get from one in a traditional language.

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.