I certainly agree that code is usually more readable close to when it's written, in both personage and time. What I'm skeptical of is that redundant type annotations always (or usually) make code more readable. I do believe they often do, but I think they should ideally be reserved for those cases. I won't always get it right, but that's something that can be improved with experience and especially in code review.

"Also, how happy would you be as the maintainer of a codebase if a new guy came along and submitted a pull request which was purely the addition of an already-inferred type?"

Slightly happier than someone submitting a pull request which was purely the addition of a comment. In both cases, my response is to try and understand why they felt the code less readable without it and see if that motivates other changes, but then probably to happily merge it.