When Austin turned heel, WWE had The Rock. When Hulk Hogan turned heel, WCW had Sting, Luger, and Flair. If John Cena turns heel, whose going to replace him? See your not thinking about that. Your too busy thinking of how great of a heel turn it would be for Cena and not realizing that if WWE were to make Cena a heel, WWE would lose money on merchandising and other deals and it would be a huge risk, especially in WWE's roster seeing as though Cena is their only cash cow. You say WWE had balls back then but that was in a time when heel turns were pretty much common during the Attitude Era. I got news for ya, the Attitude Era is dead my friend, it ain't coming back.
I also don't see whats the point in turning Cena heel in the first place. WWE would lose there only cash cow if he were to turn heel and WWE already has great heels like CM Punk, The Miz, R-Truth, Christian, Del Rio, Cody Rhodes, and Mark Henry. I just don't see the point in turning your only cash cow that is Cena just because the butt-hurt IWC can't get over the fact that Cena will be the face of the company for many years to come. Just to put it simple, IT IS NOT NEEDED.