Writing code ≠ maintaining code someone else has written. Duck typed code relies in general in implicit behavioural assumption on how the values you're receiving should behave, and it's much better to be the one making choices than the once trying to understand them.
I'm not arguing that it's not possible to do it; I just think that it requires a bit of extra steps and care to document which assumptions your code takes and how it behaves that could be more meaningfully described by a statically typed language. But these are just my 2 cents.
It seems definitely promising; it depends on how much adoption it's going to take. Even the most interesting languages (like Nim, for instance) struggle these days without corporate backing.
Worth pointing out maybe that MyPy is not a language but a type checker that works with regular Python. I've been getting into the habit of using it on everything I do lately and find it extremely useful. You can even do runtime checking on the types with tools like Enforce [1], and the Molten framework that was posted on here last week [2] uses MyPy typing as a core part of the framework.
I'm not arguing that it's not possible to do it; I just think that it requires a bit of extra steps and care to document which assumptions your code takes and how it behaves that could be more meaningfully described by a statically typed language. But these are just my 2 cents.