User-defined static types are a theory of a solution. But mostly we don't know what the hell we are doing, so we don't have a correct theory. And this is right, because instead of perfecting our theory, we should be adding the next new feature or new product.

Exceptions include frequently reused code (libraries, components, frameworks) and well-specified problems (rewriting a known problem, implementing an algorithm, shuttle-like high-risk projects). Here, static types are also useful as documentation.

As Brooks said: plan to throw one away; you will, anyhow. i.e. code to understand, then to solve. You don't understand it well enough to have a correct theory the first time, and it's less feasible to rewrite a project from scratch the larger it is.