I do think that's helpful compared to the strict alternative, but much of the up-front cost is still there. You still write most things twice, and the static type checker still slows you down when prototyping. You'd still be tempted to write bad code to work around the insufficiently-expressive type language, rather than write it in the most natural way, and only give up when it's too hard.

You can also approach this from the other direction: why not start with a dynamic language for the rapid prototyping and gradually introduce typing as the code stabilizes? Python and Typescript do this.