You all make very valid points about Cena's heel turn but I think it won't happen. As far as t-shirt sales, they would go down. Would some people buy his shirt? Of course but the bulk of where they make their money, the under 10 population would not and why? Because unlike us older fans who don't have the ability to not criticize the show for the backstage info we like to think we have, kids watch and take the show at face value. Whoever is a bad guy is a bad guy and whoever isn't, isn't.
Second, the idea of Cena being heel for beating up the Rock isn't positive either. Going back to his under 10 population, most of which who don't know or care about that Rock, wouldn't care if he beat the Rock up. This idea that everyone in the world loves the Rock is just plain dumb. There are dare I say, some people out there who don't love the Rock especially the people who are pretty damn sure he won't stick around after Mania. Just like there are people who actually want Hunter to end Taker's streak (shock I know). In fact the Raw after Mania will probably be the last one he's on. Think about it, you think the WWE would waste a Cena/Rock which has been called a dream match on WWE television or a PPV like Judgement Day? Extreme Rules ect when they could have done it at Mania? HELL NO.
Third, it's not so much about Cena's t-shirt sales as it is the way WWE has marketed him. He is the Make-A-Wish Foundation Ambassador, you think WWE is going to jeopardize the relationship they have built and Cena has made with the kids for a heel turn that as someone said, isn't needed. Could he still do that as a heel? Sure but why would they? Can he even be the ambassador when the kids stop "wishing" to meet Cena cause he's not a good guy anymore? The WWE is going to please their newer generation of fans over their older, why? They know the attitude era fans are getting older and on their way out! Because the 30 year old man who is watching is not going to spend over 100 bucks on merchandise, isn't going to pay 40-45 bucks for every ppv and go to 3 or 4 or more shows a year. It will be those 5 and 6 year old fans of Cena's that will be doing that stuff. And those 5-6 year old fans who will still be watching in 10 years. WWE wants to get the parent watch groups off their back about how dangerous, aggressive wrestling is, so they went PG. To stay PG, they need a role-model for the kids. That is what Cena's gimmick is for. From what I have seen the only wrestler parent's do approve of is John Cena. It isn't for us it's for the kids, never back down, never quit. No matter how likable Randy Orton is, no one is going to want their children to "look up" to Randy Orton or many of the other babyfaces.
One last thing is that Cena would have to WANT to turn heel. He has a lot of pull in the backstage area "supposedly" so I would assume he would have a say in whether or not he wanted to do it and from what I can tell and what Cena has always said. He could care less what people think and he doesn't have to. Unless viewership actually goes down because of JOHN CENA and nothing else but John Cena, they have no reason to change. Cena fighting Morrison, fighting Truth, whoever isn't enough to make him a heel, faces fight eachother all the time, no one cares anymore, they will cheer for who they want. The only real way for him to be heel is if he talks crap about the fans and face it, THAT WILL NEVER HAPPEN.