It doesn't work. The guy wins everything, or loses in a way that doesn't put him down at all, and he is pretty much hated by mostly everyone except for a few women and lil jimmies. A wrestler shouldn't keep the same gimmick over 10 years and have it written that he wins everything. It kills the development of everyone else as well as the product.
WWE is out to make a profit. They profit off of wrestlers who people care about. That includes if you hate the wrestler, because the more people who hate him will create more people who love him. Cena has more than just women and children - you obviously don't pay much attention to audience reaction.
He doesn't "win everything" either. He hasn't been a belt-holder for well over a year now. Stop exaggerating.
You can deny it all you want. Cena sells because people have opinions on him. I like Cena for the simple fact that he pisses people off. He's not the greatest in-ring talent, he's not the greatest promo cutter, he's essentially Hulk Hogan all over again - but it drives you guys nuts and that's just freaking hysterical.