[quesera]

You're right, DH might be enough, depending on goals.

The DH exchange would be MITMable, but not passively collectable. TLS is (ideally) neither, so DH wouldn't provide an equal level of privacy.

Still, it would be a beneficial extension of the protocol. At the cost of an additional TCP RT.

[jimktrains2]

Right, however, the MITM attack would also come at the cost of causing the rest of the connection to fail. You could also do fun stuff like sending the sha256-mac of the hostname using the DH key as the MAC key. There are lots of fun ideas!