^Burden of proof. We have several pieces of evidence pointing to not-Khan. We had only one statement from a cast member suggesting Gary Mitchell, with no corroboration. We have zero evidence pointing to Khan, because rumors are not evidence. Given a choice between multiple conflicting hypotheses, the one that has several corroborating pieces of evidence (including the actual name of the character in official press releases, for gods' sake) is obviously preferable over one that has only a single unsubstantiated claim or one that has nothing but rumor to support it.
Rumors of Khan aside, I keep coming back to those Starfleet sideburns Cumberbatch has. OK, you can put a uniform on Khan, it was done in "Space Seed", but the sideburns? How are those explained? He was given a uniform and a haircut? They're more regulation TOS Starfleet than Pine's are, for crying out loud.
Further, I agree, denials have to be believed until real evidence points the other way. It's all rumor, and those are fun, but they don't count.