The only situation in which I could ever see him leaving Barca would be the situation we've seen in the past 2 months. There has been an over-dependence on his performances. He's thus far not had any serious time out of the squad, nor any poor games. Then you look at Munich rip Barca to shreds with Messi not performing in any meaningful way.
If Barca can get 150M+ for him, and sign 2 or three of the best center forwards immediately beneath him in quality, they'd be crazy not to. I'd sure as hell trade him for Falcao, Cavani, and a few other players (plus cash).
At a certain point, they're going to realize they have all their eggs in one basket. Its a gift and a curse, as he'll win almost every game for your team. A Munich semi-final comes along and he's injured, and you go out 7-0 in absolute humiliation, almost entirely due to an over-reliance on a single player.