It would be the equivalent of Austin going heel. As soon as he did, business went down and the Attitude Era was over less than a year later. Vince hasn't forgotten that.

Not only is Cena the WWE's top draw, but now that Rey Mysterio's career is all but over he's the only wrestler that draws a unique demographic of kids and women. He's just as important to WWE outside the ring as well as inside, and has been long known to be the workhorse of that company. Any time WWE has some sort of event, 19 times out of 20 Cena is right there, and he hardly takes any days off.

It makes no sense jeopardizing theirs as well as his marketability by turning him heel. A slight tweener, maybe, but never full heel. And the logic of turning him heel is only made worse when you consider that WWE's business has been on an upturn in the past year thanks to Cena's feuds with Lesnar and Rock.
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