There are downsides. But the upsides outweigh the downsides.
I'm not a consumer of APIs. I've done game programming, robotics, embedded system development (with C++ and rust), (web development frontend with react/without react, with jquery, with angurar, with typescript, with js, zod) (web development backend with golang, haskell, nodejs typescript, and lots and lots of python with many of the most popular frameworks with flask + sqlalchemy, django, FastApi + pydantic, )
I've done a lot. I can tell you. If you don't see how types outweigh untyped languages, you're a programmer with experience heavily weighed toward untyped programming. You don't have balanced experience to make a good judgement. Usually these people have a "data scientist" background. Data analyst or data scientist or machine learning engineers... etc. These guys start programing heavily in the python world WITHOUT types and they develop unbalanced opinions shaped by their initial styles of programming. If this describes you, then stop and think... I'm probably right.
You are wrong. I learned programming mostly in C++ in the late 90's, and programmed in C, C++ and Java in professional settings for a decade or so, and still do from time to time.
Hm if you want or have time, can you give me a specific example of where no types are clearly superior to types? Maybe you can convince me but I still think your opinion is wrong despite your relevant experience.