True enough, but unless there is some way humanity is going to jump immediately to perfect protocols, then then the tradeoff between backwards-compatibility and

True enough, but unless there is some way humanity is going to jump immediately to perfect protocols, then then the tradeoff between backwards-compatibility and downgrade attacks was always going to be with us. The mere lack of any kind of encryption in our basic protocols can be considered the most important "downgrade attack" around since we never are quite sure when something is going to leak out into the huge background of plaintext.