I do not see Messi burning out anytime soon. Think about the differences between him and Ronaldinho. Messi is pretty much injury-proof (knock on wood), he doesn't drink or party much, he lives the life of a strict athlete in terms of a strict diet and rest, and his in-game style is completely unique. The better Messi has gotten, the more he has been walking around the pitch. He is saving his energy for short bursts when he needs it and this is REALLY good for his overall endurance and prevents him from burning out. I doubt he will lose much pace by the time he's in his 30's and frankly nowdays, he doesn't even rely on his pace to score. Relying on pace for scoring is a feat of pure physicality and brawn which has huge limitations (i.e Torres). The reason Messi is so out of this world is because of his incredible footballing brain and uncanny knack for goal.
Also, the whole thing that Messi is sucking the life out of the other forwards doesn't seem to sit right with me. I mean think about the difference between the old lineups with messi and the new ones. Alexis-Messi-Pedro has one goalscoring forward and 2 natural wingers who can't finish. Henry-Messi-Eto'o has 3 goalscoring forwards and all 3 of them were elite finishers. Messi is scoring a lot of Barcelona's goals and this can be construed as "Messi-dependancy" but that's mainly because of the attack options. I remember seeing a lineup in the argentina NT with the attack structured as Aguero-Higuain-Messi. In that game, all 3 of them scored. That adds proof to the truth that while Messi does score a collosal amount of goals, he doesn't do so by starving the other forwards. The reason Alexis and Pedro aren't scoring is because even though they get at least 1-2 scorable chances per game, they hit it wide, at the keeper, or it gets blocked by a defender.