> slowly become statically typed

They don't. They become gradually typed which is a thing of it's own.

You can keep the advantages of dynamic languages, the ease of prototyping but also lock down stuff when you need to.

It is not a perfect union, generally the trade-off is that you can either not achieve the same safety level as in a purely statically typed language because you need to provide same escape hatches or you need a extremely complex type system to catch the expressiveness of the dynamic side. Most of the time it is a mixture of both.

Still, I think this is the way to go. Not dynamic typing won or static typing won but both a useful and having a language support both is a huge productivity boost.