This whole "turn Cena heel" thing has been going on for a long time. What seems to be getting forgotten in all of this is that turning the top face on the show heel is rarely successful, thus it's always a huge gamble.
Bret Hart was the top face in WWF and was seen as being stale by a big chunk of the fanbase. Stone Cold Steve Austin was the top heel but was so charismatic and good at being a badass that he was starting to be cheered by a big chunk of the fanbase. At Wrestlemania XII (if I recall correctly), the two face off, wrestle a classic match, and Austin passes out in the Sharpshooter rather than tapping out, thus proving himself to be "the toughest SOB alive". After the match, Hart goes on to become a tremendous pro-Canada, anti-USA heel and Austin goes on to be the biggest babyface wrestling star in history. You have to remember that neither of them really changed their character much, outside of toning down certain aspects and playing up others. This was a huge gamble on the part of WWF, but it worked.
Hulk Hogan was the top face in WCW. Kevin Nash and Scott Hall were on their ways to being the top heels, with the Outsiders and the mysterious "Third Man". Suddenly, the third man is revealed to be Hogan and the nWo is born. This goes on to be one of the most popular (some would say the most popular) heel stables in wrestling history. This was a huge gamble on the part of WCW, as you never know how a crowd's going to react. What if the crowd had cheered Hogan, Hall, and Nash? Again, a huge gamble, but it worked.
Stone Cold Steve Austin faces off against the Rock for the WWF Championship at Wrestlemania X-7. At the end of the no disqualification match, Vince McMahon (evil billionaire) helps Austin win and the two celebrate by downing "Steveweisers" in the ring. The crowd had no idea what it was supposed to do and the majority of the Houston, Texas, crowd (as well as the San Antonio, Texas, crowd the next night) continued to cheer the "do anything to win" ethic of the Rattlesnake. It was a gamble trying to turn Stone Cold heel and it failed miserably.
I bring these up to say this: you can never be sure how a heel turn or face turn is going to work out. WWE chickened out last summer. They should have kept Punk away for a while, let the title situation stew, with a renegade WWE Champion out there, building up to unification match at Survivor Series (think of the fantasy booking, sending up sacrificial lambs to face "Corporate" WWE Champ Cena while still finding legitimately respectable competition against "Renegade" WWE Champ Punk, matches to be held on WWE PPV's, as that's the place where a champ needs to defend his title....now end the feud at Survivor Series with Cena doing "everything it takes" to beat Punk, cheating his way to victory to become the "True" WWE Champion, turning heel and getting more pops for Punk, thus not compromising the things that made Punk's character fresh and exciting and reinvigorating Cena's character, also allowing Punk to "re-sign" with WWE in a quest to get his title back, winning the Royal Rumble, qualifying for Elimination Chamber....the possibilities rapidly become endless, but, no, they didn't do any of this). But it was too big a risk.
I can understand WWE's reticence in its handling of a Cena heel turn (hell, look at the muck-up made of Punk's face turn; even his fans didn't like it). They have to decide if it's more important to take the risk and try something new or if it's too expensive to even attempt. Unless TNA or ROH becomes big enough to be an actual threat, I don't know if they'll ever have that kind of courage again.
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