I'm tired of Cena, and have been for a long time. There's been quite a few opportunities for them to switch him, just as you describe, but they won't do it. T-shirts, merch, Make-A-Wish, etc, etc. They don't have anyone else who can become that poster guy right now.
I will make the argument that the last wrestler (not including Vince) to get good heat and be a good heel was Slaughter when he was doing the whole Iraq gimmick around Wrestemania 7.
I'll throw JBL in there, he was an awesome heel, and I'm sure I read a story about them having to sneak him out of somewhere in Mexico, 'cos the crowd were going to kill him?!