Thinking so strictly about what types a function will receive rules out making use of things like decorators in python that operate on function arguments regardless of type.
Or generic validation utils that take many types and return the passed in type after performing internal validation logic relevant to the type.
Those generic use cases are good examples of flexible types, but I have a hard time seeing how
* "I don't know whether the `getMyReplies()` function returns a list of items, an iterator, a non-rewindable generator whose results I need to cache, or a closure with/without memoization" and
* "I don't know whether the singular Reply entity has a field called pid, parent_id, parentId, ParentID, ParentGUID, or parent_message_identifier"
are advantageous to rendering a list of replies.
Languages like C and C++ made it readable with debug symbols.
The web stack does the same with source maps. The TypeScript compiler also offers to emit .d.ts files with type information for reuse in other projects.
Treating the code as statically typed makes the code predictable, less prone to bugs, and easier to maintain. If a function's return type is determined by dynamic factors at run-time, it becomes a maintenance and debugging nightmare; the effect is compounded as more and more unpredictable dynamic functions are chained together.
> operate on function arguments regardless of type
parameterized types and other techniques allow you to describe this type of behavior in a type safe way.
> Or generic validation utils that take many types and return the passed in type after performing internal validation logic relevant to the type.
All these things are possible in typed languages.
Both the things you mention can be done in statically typed languages. The only difference is in dynamic languages you don't have to declare interfaces, subclasses, etc. I just wonder whether that really is as much of an advantage as people think.
I've spend years using both types of languages and I think I think the safety of statically typed languages is usually worth the slight amount of friction it adds.