Of course the idea is to change the overall direction and you use the biggest star you have as that catalyst.
If people thought the WWE was capable of being innovative enough to take on this direction shift, then they'd overwhelmingly vote to turn Cena. But, and I think rightfully so, people assume WWE is so risk-averse that, if they turn Cena, there's no way they'd have the balls to also ramp up their product's edginess and use a Cena turn to usher in a new era.
Right now, WWE is a bland, blatantly-scripted and mostly boring ballet/infomercial. I feel like if they turn Cena, it would stay that way -- only Cena would maybe do a few things differently so he can have a generic heel label slapped on him.
It kind of goes back to what you briefly touched on on your last podcast. When you're analyzing this stuff, you can look at it through the prism of what you would do if you were in charge, or try to come up with ideas that could plausibly fit into what has a realistic shot of actually happening in today's WWE.
Right now, it's totally unrealistic to expect that the major direction shift that would need to accompany a Cena heel turn would ever happen. Most people realize that, and are hesitant to turn Cena.