SMTP Strict Transport Security

Doesn't that just break compatibility with existing servers if clients start blindly refusing to communicate with servers unless they have SMTPS/STARTTLS, without caching some minimum expectation of the security features that the server should be providing. You want clients and servers to be able to negotiate and speak any generation of the protocol, without renegotiating every time as that opens you up to trivial protocol downgrades.

A better question is why not just have clients cache the highest level of protocol you've detected for any particular server, and not allow negotiations to any lower version of protocol, my guess is that this has been thought through more clearly by the implementers who found that explicit notification from the server to engage this behavior fixes some corner cases. Many servers who offer STARTTLS today do so with invalid or expired certificates, I'm not even sure it's well defined what should be verified in the certificate to validate a connection today, you would not want clients to reject invalid certificates deployed today, but you would want to reject invalid certificates if the server owner has signaled that they expect them to be valid. Even better is that this policy signalling is out of band, via HTTPS or DNSSEC, that is harder to MITM today.