You seem to be focusing just on the (admittedly problematic) problem of parsing outside and possibly invalid input. This is hard, especially if the API is poorly defined.

But this is hard with any typing system. Using dynamic typing will just hide the problems under the rug, and they will explode in your face later on. Static typing just made those problems explicit.

There are many benefits to static typing, beyond the external boundaries of the system, where it definitely lives up to its promises.