I'd say the chances of Rock winning at Mania are extremely slim to none, and I can't stress how slim it actually is. The only reason I'm even considering the possibility due to the fact I didn't expect him to win last year (although circumstances are very different here).
Anyway, the explanation. The story behind their Mania encounter is at its core, about John Cena regaining loss success after losing his bigger match by winning an even bigger one. Cena placed his match with Rock high on a pedestal last year, to the point of saying if he lost, his career meant nothing. He ended up losing it, and he began his fall. Then he beat Brock Lesnar and went on to have a pretty damn good year . . . but in WWE's mind he had a fall. The beginning of 2013 lead to second chances, and at the Rumble, unforeseen (kayfabe) circumstances lead to him being able to get the rematch of a lifetime. Because remember, it was Once In A Lifetime because they thought at the moment it would be. Cena wouldn't have placed the match highly if he knew then he'd just get a rematch but I digress. The point is now Cena has found himself and the same position as last year except this time the stakes are higher, the WWE Championship is on the line. He beat CM Punk, the other opponent he couldn't "beat" (Just go with it; you know with this and the fall "Just go with it" could be the entire motto of this feud) and is now going to redeem himself at the grandest stage of them all. And that is the reason he'll win, because the match happening at Mania is a big plot point in the story. WrestleMania is without a doubt a much more important show than any that WWE produces. Its the biggest show, its a big reason to why Cena placed that match so high up on a pedestal. Him redeeming himself, involves him beating The Rock, but it also involves him beating The Rock in the biggest match in his career. A rematch after two failed attempts at Extreme Rules is not the biggest match in his career. The rematch of a lifetime at WrestleMania for the WWE Championship is the biggest match in his career, and from a narrative point of view is the place where his victory will have the greatest amount of satisfactions. The story hinges on the fact that Cena beats him at Mania when everything is on the line, it's not just about beating him. Yes, you could also say that the story hinges on Cena taking a fall but WWE has always been a lot better at booking Cena as a winner, not a loser, so the same faulty booking isn't likely to happen here.
Extreme Rules will be the rubber match and I prefer it that way for several reasons. A big one being that in the scenario where Rock wins at Mania, why would Cena get a third chance to face him? And I mean a legitimate reason, not a, "He's John Cena".
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