I think he is, without question, a legend. With all due respect to a previous poster in this thread, he is more than the face of the company. He has been staple, and the catalyst for many, if not most of the major events in WWE in the last decade. You could easily call Punk or Orton faces of the company, but neither have achieved the success, fame, or accolades that Cena has. He has grown to the level of a Hulk Hogan, Rock, or Stone Cold Steve Austin, in that he now transcends professional wrestling. He is a household name to many that don't even associate closely to wrestling.
When you look at his body of work as professional wrestler, his amount of title reigns, the lengths of some of those reigns, coupled with the feuds he has been involved in (and make no mistake about it, his current feud with Rock is huge brick in the foundation that cements his status as a legend), he is the biggest name in the business, and has been for many years.
Whether loved or hated, he's carried the company on his back for years, and has been perhaps WWE's best publicity and good will campaign machine since the days of Hulk Hogan.
His 'era' will not be forgotten in the WWE. You don't reach the level of success that Cena has, to find yourself forgotten. That is as absurd as saying WWE has forgotten about Hogan, Austin, Flair, or Rock.
The biggest thing working in his favor is that, by all accounts, he still seems to have several, if not many years ahead of him in the WWE. His popularity and lack there of, as well as his merchandise sales, willingness to promote the WWE, Make A Wish, and everything else he does, ensures his spot at the top of the heap for as long as he chooses.
He will never be seen as the most gifted wrestler, and he is probably the most polarizing of any active 'legend' in WWE's history, but his contributions and status can't be denied. He's been given an entire year to build just ONE match with a superstar already easily considered a legend...if that isn't a nod by WWE to cement his status and to affirm his place as WWE royalty..nothing is.