The answer is right under WWE's noses, but they can't see it...
Beating John Cena MORE THAN ONCE is the instant ticket to legitimizing someone. Now, that's just an example - it shouldn't be Justin Gabriel. But it isn't coincidence that Punk became a top start the moment he beat Cena, then beat him again. Randy Orton peaked when he was going over Cena. Wade Barrett was at his most over the night after Hell in a Cell 2010, when he beat Cena. The same is true for others.
Cena will be a top star no matter what, but if you borrow some of his heat and transfer it to someone else, they'll be over. Imagine how over Dolph Ziggler would be if he beat Cena two or three times.