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?
Well it doesn't matter one iota if he "is the true Mahdi" or just a skilled poseur, if 100s of millions of death cultists believe he is the Mahdi, then he IS the Mahdi.
One more possibility: put a close double out in public, then shoot him in the head with your own Al Q'aida sniper. Do it on TV. Blame it on the CIA.
Then, have the real OBL show up a few days later with a bandage on his head.
How's that for myth making?