He is incredibly stale though. Big changes do wonders for careers and fans of the product. Kids do make up a lot of the WWE audience, this is true, but there are still a lot of teenage/adult fans who watch that are sick to death of Cena. During his heel run and even early stages of being a face as the Dr. of Thuganomics, Cena was insanely over with kids and adults. Turning him heel would do about as much good as it would "bad" for the company and his character. All big faces eventually turn heel. SCSA, The Rock, Hogan, Flair. They've all done it and their careers became much better.