HTTPS is already designed with that kind of decoupling in mind. But it wouldn't make sense to offer encryption without identity verification to the end-user, because that would make the encryption useless, so any protocol that does encryption has to do both.
I know that. That's basic AAA.
Also note that for an effective MITM attack you would need to have new certificate for which you have got the private key. There are a number of things that will make this increasingly difficult in the future, like certificate pinning, increased willingness of browsers and OS vendors to blacklist CAs, and increased monitoring for rogue certificates which makes it easier to find rogue CAs.
I think you fail to realize the scale, the proportions, of the opposition the browsers face.
It's not some script kiddies who are threat here.
That's countries covering close to a half planet's population. They might as well simply outlaw the browsers. In fact, they already do outlaw some encryption software.
I personally would still argue that the CA system is the Achilles heel of HTTPS but the situation is getting better and it's a matter of time until we get a more distributed and robust way of certificate verification.
But that's another problem: you can't make CA distributed. CAs are the "single point of failure" which are allowed to be that, based on the promise that they will work hard not to fail. Making it distributed would basically nullify the promise, making the whole CA system vulnerable. IOW, nothing changes.