The main problem with this is that STARTTLS is not anywhere near good enough, but if it sees high adoption, nobody may bother with something better in the future because they'll all think "Mission Accomplished."
There is never something that is "good enough" to solve all problems. In addition, stronger approaches are often harder to adopt (which is why they often don't get adopted). This effort to encourage STARTTLS everywhere provides reasonable protection against bulk passive monitoring, at very low effort. I think encouraging the use of TLS for all email traffic is a good idea.