If Rock won, who would eventually beat him? After beating Punk at least twice, he's not losing it to him and after coming off such a long reign, I really don't care to see Punk win the belt again anytime soon. Guys like Ziggler or Bryan or whoever beating Rock just isn't realistic (at all...), and they're not at the point where I'd want to see them do it anyway. Cena is the only guy I can see being put over Rock at the moment.
Yeah, it makes the outcome of it all a bit obvious, but it also seemed obvious that Cena was winning the Rumble, Rock was winning the title and that Rock/Punk would go on last at the PPV, but all three still happened. I wouldn't look forward to any other outcome to Wrestlemania than Cena walking out with the belt. And I don't even wish against it, because Cena losing to The Rock again would almost confirm that Rock/Cena III is happening, because there's no way Cena isn't getting his big win over Rock at some point.