1: It actually won't make money. Cena sells a lot of merchandise to kids who worship him as a hero. Superman became a bad guy mid-story, he wouldn't be the superhero icon that he is today. As cheesy as it sounds, Cena is supposed to stand for good. The one guy kids can count on to not be a selfish bitch like the other ones.
Two words: Hulk Hogan. Hogan's act was far more stale in 1996 than Cena's is now, granted, but it could also be argued that things move much faster now than they did back then because of the overexposure of the product.
It's all about target audience. If the WWE wants to continue having kids and families as a primary target, Cena will remain a face. If they want to make the product more adult oriented, he could turn heel and it could very well make buckets of money.
Not that I think it will happen anytime soon, mind you... but if it did, I'd do it at Royal Rumble by having him cost Rock the title... yes, we'd have to have Cena vs. Rock 2 at WM, but with a heel Cena it would be fresh... and besides, we all know Cena/Rock 2 is happening regardless.