The fact that Cesc is under contract till 2015 means we have him for another four seasons. Obviously Wenger won't want to let him go for nothing, but I still think the little we'll be losing each season he gets closer to being free to leave, we more than make up with his services each season.
Having said that, I'd expect him to go in the summer of 2013, as leaving it to the summer before he's a free agent would just be too risky as he'd be so close to going for nothing.
Even so, if Ramsey can get back to his best of last season, and even improve on it, I see no reason why Cesc leaving would cause an irreparable hole in our team in two seasons' time. With a midfield of Song, Wilshere and Ramsey, with players like Nasri able to play there, and Eastmond, Frimpong and Lansbury coming through.....