This whole situation kind of blows. Punk has to lay down to a star of yesteryear who won't even be around after Mania, and the payoff to this whole thing, as it always is, will be Cena coming out on top over everyone else when he doesn't even need it. Fucking fantastic.
The way I see it, though, Punk losing the WWE Championship to The Rock at The Royal Rumble (theoretically--since it's almost surely going to happen) is probably going to free him up to wrestle a "special attraction" star of yesteryear/part-timer in either Stone Cold Steve Austin or Triple H, and I suspect that he will win that match at Wrestlemania.
Punk's going to have to drop the WWE Championship one day, and in terms of kayfabe there's nobody who's higher than The Rock. They refer to him as The Greatest of All Time these days. Rock defeated Cena.
Now, of course, Cena's going to win the rematch vs. Rock at Wrestlemania, but if both Cena and Punk win their respective matches at Wrestlemania against Rock and Austin/Triple H, then I'd say it is a case of overall good booking with a few hiccups along the way.
I'm hoping they don't throw the WWE Championship onto Cena between now Wrestelmania, though, so that Punk can indeed have a 14-month-long reign and that Cena's match for the WWE Championship against The Rock actually has true meaning behind it. Giving him the belt for a courtesy reign since he hasn't had it since the first few days of October last year would be a horrible move. They have the chance to almost replicate the storyline arc of Steve Austin's against The Rock going into Wrestlemania X7--not with the heightened drama based in reality that Rock was usurping Austin's top spot in WWE while Austin was away due to the neck surgery, but in terms of the length of time between Cena's last championship reign and the Wrestlemania to take place a year and a half later. Unfortunately, I'm expecting them to screw it up like they almost always do, especially when Cena's involved.