While I don't expect John Cena to turn heel anytime soon, whether it be associated with Nexus, or via an unrelated heel turn, I don't get the notion that he won't because it will hurt merchandise sales, it will hurt Linda McMahon's Senate race, it won't jive with the PG era, etc.,
Historically there's been lots of WWF/WWE superstars who have been extremely popular, and almost all of them, if not every single one of them, has undergone a heel turn at some point in their careers, and usually a swerve one. Hulk Hogan turned heel, albeit with WCW rather than WWE. Stone Cold turned heel. The Rock, the Undertaker, HBK, Jericho, Bret Hart, the list goes on and on. I imagine these guys have moved a lot of merchandise in their careers, yet their heel turns did not cripple the financial empire known as the WWE and bring it to its knees because they didn't sell as many T-shirts.
So why is it that a John Cena heel turn is so impossible in most people's eyes? While I don't see it happening at Summerslam this year or in the immediate future, in all likelihood it will happen at some point, it always does. Even if it just for a very short while before he turns back, it will happen some day. Count on it.