Kayfabe: I feel like Cena has put away just about every big bad. Cena versus The Bloodline feels like a match he could almost win alone.
Backstage politics: Cena hasn't ended many notable streaks. He didn't take Undertaker's WrestleMania streak, even though he very likely could have in kayfabe. He lost to RVD at One Night Stand. He's able to respect other's storylines and put others over (Edge, Orton, Triple H) if they have enough star power. Roman is a big bad, the ultimate storyline. In a world where Cody wasn't around and Cena was on the main roster, would he further the storyline in hopes of an even bigger star or would they have made Cena champ?