OP draws a false one-dimensional relationship between types vs tests in terms of code quality. Writing expressive types instead of tests does much more than affect a quality curve - it changes the way you approach the problem you are trying to solve. The classic Haskell example is understanding how IO being a monad allows you to push impurity to the edge of your system.

Start-ups decide not to write MVPs in languages like Haskell or Idris not because those languages aren't "rapid" enough, but because it's too difficult to find programmers experienced in those languages on the labor market. It's already difficult enough to find competent programmers - no founder wants to make their hiring woes even more difficult.