Another comment that just occurred to me: JavaScript becoming gradually typed is an interesting reflection of the recent history of the optimization of JavaScript interpreters, which consist of deducing where semantically dynamic objects behave like static class instances, then inlining the accessors and where beneficial, the "class methods", and specializing && JITing the semantically dynamic functions that almost always take as argument "instances" of this "class".

(ref this absolutely fascinating paper

http://bibliography.selflanguage.org/_static/implementation....

and this piece of V8 dox quoting the aforementioned paper

https://developers.google.com/v8/design)

It seems that adding a type system to a dynamic language has little real drawbacks compared to designing language and type system at the same time, for both performance and type safety considerations.