You can't look at 40-50 starts and just expect that's going to be his average throughout his career. He came into the league skinny and throughout his career he's been bulking up and that takes some time to get used to. Bynum is really LA's only big body capable of defending guys in the paint (Duncan, Shaq, Nene, Perkins, Stoudemire, etc etc). LA already has Bryant, Gasol, and Odom who need to have the ball in their hands to get theirs, and adding another to that isn't going to make them better, and they'll lose a lot of efficiency offensively because Bynum doesn't command touches, gets double teamed, and is one of the few pure low post players left in the NBA. Bynum is only going to get better and better while Bosh has hit his peak. Right now Bosh is a 2nd option on a good team, 1st option on a bad team, while Bynum could contribute as a secondary or tertiary scorer on most teams right now, and is going to be one of the best post players in the league.
Sorry, honey, but role playing is part of the game. The Lakers do not need any more guys who need to dominate the ball to get theirs. They need guys like a Bynum, a Perkins, a Kirk Hinrich, etc etc, who will come in, not demand the ball, play top notch defense, and be efficient with their touches.
And yes, he's 22, he's a kid.