I considered that as well, but it seemed to me that other software on the network (say, different wallet implementations) might prefer the option of being able to bump up their protocol version in the future to pick up some other change, without having to also opt-in to receiving headers-announcements for blocks.
In particular, inv-based block announcements aren't going away (even in my implementation of headers announcements, there are some edge cases where the code would need to fall back to an inv announcement), so forcing all software on the network to upgrade to supporting headers announcements, whether now or in the future, seems too drastic -- I could imagine some software not being very concerned about optimizing block relay in this particular way.