I would say that Elm lets me refactor front-end code fearlessly. I can confidently rewrite abstractions that are so fundamental that I'd be stuck with them forever in a dynamically-typed language because the rewrite would be so much more costly.

It reminds me of when people say that Mongo is better than Postgres for prototyping because it doesn't have a schema. In my experience, the very concept of having explicit schema transformation as the schema changes is why I'd consider Postgres to be better for prototyping: it's precisely the time when your schema is changing the most.

Of course, if we could unanimously agree on these trade-offs, then we would all be using the same stack and there would be no dynamic vs static typing debates.