It makes perfect sense, because you do all you can to win while Kobe is in his prime. Bynum will still be a nice player in 4 years when Dirk and Kobe are done, but getting another Kobe (which you almost always need to win a le) isn't guaranteed, and surely won't happen right away.
Dirk's not Glenn Rice; a guy who is going to need double screens all night. He's a fast bigman who can murder opposing frontlines with the fouls he draws, and he can get his shot off over anyone. Dirk and Kobe on the pick and roll would be the most unguardable play in the NBA by far.
If Kobe was 3 years younger, maybe I could see keeping Bynum instead, but with an aging star you go all out to win now.