Look at it this way. Cena is already the top guy in WWE, and it's not even close.
The only point to turning him heel would be to have him eventually lose to someone else so that someone else could supplant him as the top guy in WWE. Assuming that's even how the storyline ends. Jakyl should be all over this one, as he correctly predicted the ending to Immortal pretty early on.
Who's to say that the dominating Cena-led heel stable ends up collapsing because Randy Orton or (god help us) HHH reaches out and brings "the real John Cena back" and Cena turns on his own stable?
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SmartMuffin - Because anything less would be uncivilized - http://img.imgcake.com/smartmuffin/barkleyjpgde.jpg
http://dudewheresmyfreedom.com/