Imo optional typing like this is the best trade-off for now.

Usually, statically typed languages is not expressive. There are a lot of abstractions are hard to express in statically typed languages. Even in Haskell. With optional typing, you can express usual ideas with types, and cheat with 'any' when not applicable.

Another thing is the language itself can possibly execute without compile or even type check. Making developing feedback cycle much smoother when running tests or just save and refresh.