Anyone can be "top heel" if that's how they're billed.
Was Jericho billed as top heel when he was feuding with HBK? No, his character change along with his feud with HBK helped that. JBL was top heel but the IWF gave Jericho the title top heel. Triple was billed as the top face, but he wasn't, Jeff was. CM Punk was billed as an underdog, but IWF know he wasn't, that he could have had a better title reign. Jack Swagger was billed as the undefeated All American American, wins the title, and becomes losing guy who's the champ, but he's still billed as top dog of ECW. Cena was billed as the top face dominate fan favorite champ, but the fans turned on him. That doesn't sound like top fan favorite champ to me. Kozlov is billed as the dominant undefeated Mozkow Mauler, doesn't seem like he is to me though.... Should I continue of have I proved my point?
End of the day Cena sells shit makes untold amounts of money as a face. Why would they change that ?
People want Triple H to turn heel when he has good merchandise sells and the DX reunions draw ratings, that can't happen when he's heel. He can't team with HBK when Shawn's a face and him a heel. People say they want CM Punk as a heel, why would they do that when he has good merchandise sells and a huge fan favorite? Hogan was a mega face, had all the characteristics that Cena brings, and he had a successful heel run. Austin was the same as well. Why can they have heel runs and Cena can't?
Even if he goes heel it wont be Heel like it was back when. So who cares Heel isn't the same as heel was in yesteryear. With all the kid friendly pg thing. You ain't gonna get the fuck you heel of the attitude ere. Or the Cena heel like he was.
So Edge, Jericho, Orton, JBL, Miz and Morrison, Kendrick, Zeke, Swagger, for example can be heels, but Cena can't because it's not the attitude era (which I don't think Cena was even part of, he debuted in 02 I believe). I'm not saying he's going to turn heel right now, not saying he's going to be the same heel as he was before but that still doesn't mean he wont be a great heel.