It's unbelievable to me that anyone could attempt to defend the stance that WWE dropped the ball with people like Carlito, MVP and Shelton Benjamin. They were given several pushes each and failed to capitalize, because they couldn't. They didn't have the natural skill set it took to be a main event level talent on the global stage that is WWE. As LJL said, they each have charisma equating to that of dirt. Same goes for that overrated walking spot fest John Morrison. It's asinine to believe these people should be mentioned in the same breath as John Cena. Cena has as much charisma as almost anyone in the history of the business. Cena can rightfully be mentioned alongside names like Hogan, Flair, Rhodes, Austin and The Rock, and with great reason. His promos are clear and concise and have incredible cadence. They "flow." And he delivers them with conviction, which in turn, evokes an emotional response in his audience - meaning he CONNECTS with them. People like Flair and Hogan and Rock CONNECT with the audience. People like Shelton Benjamin and John Morrison can't and don't.
As for life after Cena, WWE wouldn't miss a beat. They're a brand that has had sustainable success after employee attrition - even when the person is of paramount importance to anyone else on the roster (I.e. Hogan or Austin). There would be a transitionary period where people like Punk and Bryan would play an intricate role, until ultimately the next "total package" that people like Rhodes, Austin and Cena represent comes along to champion the mantel of "The Man" of that particular generation.