It's fairly obvious that for the any public facing API, type signatures are going to be a net positive, because you have to write input validation for your API

It's fairly obvious that for the any public facing API, type signatures are going to be a net positive, because you have to write input validation for your API anyway to have any hope of it being secure. You can either write as a series of buggy, hard to understand if statements or use a well-defined schema tool of some sort. Furthermore, once you have the schema you can generate client APIs in e.g Typescript for your APIs instead of wasting engineering effort on writing them manually. In terms of static types generally, outside of APIs, the equation can be different. However, if you are creating a library that many developers will use, given the state of IDEs, your just hurting your users (developers) by not providing static types. I use Typescript in every single project not because of supposed benefit of "type grantees", but because I don't need to spend all day flipping between some developers poorly written, incomplete docs to understand how to use an API. My code editor will tell me the whole format and I can explore the APIs with goto definition, demonstrably reducing development time.