But if OBL is thought to be dead, that will only reinforce his personal claim to being the Mahdi if he later reappears on a balcony.
What if he is dead, and a look-alike appears on the balcony? Osama's lieutenants would be expected to try to keep Al-Qaeda running in his absence, that would be a good way to stir up large numbers of recruits. And how do we stop four look-alikes from getting in Pakistan?