Just a quick thought, Cena (or by extension WWE booking) gets a lot of heat for being very flippant about challenges and losing and not booking main events seriously. I agree for the most part.
HOWEVER, considering this is a weekly show we're talking about, wouldn't treating main events that way every time have eventually gotten monotonous? To wit, HHH. No one could every accuse him of not treating every title match like it was Clash of the Titans and that led to the classic, "I, uhhhhhhh, am, uhhhhhhhh, the Game,uhhhh" promos that got insanely boring, in part, because, for the most part, everyone knew HHH was winning one way or the other.
Obviously, Cena has the same problem. We know how Cena vs. X, for the most part, is going to turn out. Argue about the merit of Super Cena at another time, but the writers know Vince wants Cena to win, so Cena wins, so everyone knows Cena's going to win. So why not keep the dial at 6 or 7 until it's time for 11? If given a Strawman choice between constant Super Serious and constant Super Cena, I imagine most people are likely to pick the later. In any event, Cena has the capability to turn on the heat when it's important. So why not keep things light until it's time to get heavy?
The problem is that he never does get heavy. And he's shrugged off some of the biggest potential money-drawing angles ever, like the Brock match in 2012 and the Nexus angle, just to name two. It's great that he has the ability to pull out a money promo when he needs to, but he rarely does and it makes it all the more frustrating when you want to like him but just can't because his primary method of building up a big grudge match is to show badly photoshopped baby pictures or something equally silly.