> It's a pretty nice best-of-both-worlds arrangement

It’s also a worst-of-both-worlds arrangement, in that you have to do the extra work to satisfy the type checker but don’t get the benefits of a compiled language in terms of performance and ease-of-deployment, and only partial benefits in terms of correctness (because the type system is unsound).

AFAIK the Dart team felt this way about optional typing in Dart 1.x, which is why they changed to sound static typing for Dart 2.