Here's the thing with modern wrestling...it's more business than wrestling. The Attitude Era came about because Vince was desperate to bring something different to the masses. They had fresh characters for that time and said what was on everyone's mind. The characters such as Stone Cold, The Rock, and Mick Foley both RELATED to a certain demo.
Stone Cold related to the man who was sick of his boss and taking his shit.
The Rock related to the man who busted their ass and tried to take shortcuts to get to where they were at.
And Mankind related to the underdog who everyone told they wouldn't amount to anything.
The WWE right now has few clear cut face/heels. Orton's cheered because he's a bad ass and backs up what he says. Cena's booed because he shoved down our throats and people want to see him get beat once or twice. CM Punk gets a face pop because he's straight edge. Edge and Jericho get booed because they want to see someone kick their asses.
The WWE won't turn players like Cena and Punk heel because they sell too many items. Face it. People buy more face merchandise than heel merch. It's business and as smart and creative as it would be for Cena to just snap on the crowd, tell them to kiss his ass, go back to rhyming and thuggin' and buggin', it's not good business sense, which is what Vince is worried the most about.
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I dispute the "faces sell more merchandise" argument. Firstly, if faces ALWAYS outsold heels, then no-one would ever agree to go heel. Why would they, when they are making less money than Cena or Triple H?
Secondly, certain superstars would sell regardless. I'm sure that the Undertaker moved merchandise whether he was the heel he started as in WWE, or the face he is now. If the superstar is over, they will sell regardless. Your favourite superstar will most likely stay your favourite superstar whether face or heel, so you will buy his merchandise anyway.
Thirdly, the highest selling T-shirt in wrestling ever was not "Austin 3:16" or "Hulkamania!" but the "NWO" T-shirt. NWO were a heel faction, so why did they outsell all others. The reason is because they were "cool" heels. The same fans cheering heels will buy their merchandise, because they may consider Edge, Orton or Jericho cool. Cena doing his rap gimmick was considered "cool", so he may sell just as well.
Finally, if the WWE had good business sense, they would turn people like HBK or Cena heel, because it will create fresh programs and give them fresh opponents. This is always good for business, as it gives more variety, and stops the product getting stale from seeing the same people fight over and over, because they don't turn people.
Also, about not cheering "goody-two-shoes". Ricky "The Dragon" Steamboat, when he wrestled, was the ultimate babyface, and never turned. Yet he was cheered just as loudly at Wrestlemania, Raw and at Backlash, as he always was. So "goody-two-shoes" babyfaces, who produce great matches will always be over.