The argument against this is that a URL starting with https means something, at least to some users and your proposal would undermine that. When I want to go to my bank, I type chase into the address bar and choose one of the suggestions that starts with https and then go about my business. Maybe I should be paying attention to whether or not there's a green lock in the upper left and maybe I'd notice if it were missing, but I don't intentionally look for for the lock. In your proposed change, the absence of a lock icon would be the only way I'd notice if my connection were being MITM'ed. So, in that way it would be making the web less secure for users who are entering an https URL and expect it to be secure.

That's what's attractive to me about Firefox's opportunistic encryption proposal. It really makes no change in the UX compared to an unencrypted connection, not in the URL and not in the time to connect.