Types are inferred even when you don't specify them; if you are using a library that has type specifications a lot of your code is going to be well-typed just as a consequence of using that library.
1) "Someone else wrote them" doesn't make them free. Given a limited amount of developer time, that means that the time spent on the annotations wasn't spent on other functionality.
2) If the types can be inferred, that should just happen. No objection there. But this piece is specifically about "adding (manually-authored) type annotations to JavaScript"
