... would be my choice in Afghanistan. Leaving 5,500 behind seems to me like just giving the Taliban more targets to shoot. Go big or go home. If you don't have enough troops to carry out decisive offensive operations then what's the point? They know you're going to leave soon anyway, all they have to do is wait you out. Unless you're willing to stay indefinitely, the war is lost, so get out now and stop taking casualties.
Maybe he's just doing it so the total collapse of the Karzai government happens during someone else's administration.