But even static types (even the Hindley-Milner variety) won't be of much help in a team of juniors with no technical leadership. The codebase will end up in the same place - needing a rewrite - because even a good typing discipline cannot immediately substitute hard-won lessons in program design.

In fact I never thought I'd say this - for the first two years of discovering sum types, I thought they were the silver bullet of computing, and certain experiences have sadly tempered me there :)

I still agree that types _can_ make the flow and transformation of data in the system clearer than if there were no types. Though without experience, we could very well end up with convoluted designs and that can be as difficult to untangle as a dynamic spaghetti.