There are graveyards filled with "indispensable" men.
I have seen the above quote attributed to both Churchill and de Gaulle. Whoever said it got to the crux of it: organizations are bigger than any single individual. And more broadly: everyone is replaceable.
That's not to say there aren't key players that help an organization function, make it better, or give it vision. Apple probably wouldn't be the MP3 leader today without Jobs. But it would be around in some form (it lasted for years without Jobs). That's because after an organization reaches a certain point it can still function even after the loss of a key individual. The real question isn't whether the person in question is irreplaceable, the question is "who does the organization plan to replace this person with if he leaves?"
So while I was certainly doing high-fives this morning when I heard about the death of Zarqawi, I didn't hold any illusions about the end of Al Qaeda in Iraq. This will be a Hydra that will sprout seven heads for every one we cut off. The real question is this: who is going to replace him?