I'd distinguish between the idea on the one hand that removing particular individuals from history could be expected to make a great difference, and the idea on the other hand that preventing certain key events could have resulted in a different shape to history.

I'd guess that if Hitler had died in infancy, there'd have been some other Hitler figure. It seems to me that the madness didn't have its source in the one man, but in the people around and below him. They'd have found someone else, I'm sure.

On the other hand the importance of the assassinations in Sarajevo were not that the particular individuals concerned were removed from history, but rather because the event sparked off something which might not have otherwise happened.

It's true enough that the whole international situation was like a bomb ready to be detonated, and it's possible to argue that something would have happened. But then the same could be said of the Cold War period, and it felt like that too - but miraculously we got through it without the bomb being detonated.

It seems to me that September 11 might fall into both categories. I'm sure that if there hadn't been a Bin Laden his place would have been filled by someone else. On the other hand if the on the face of it extraordinarily improbable events of September 11th hadn't happened, I don't think it's necessary to assume that something equivalent would have taken place.

(Of course the question arises whether Bin Laden as an individual necessarily had much to do with the planning of that, rather than having a sort of cheer leader, morale booster role, which to me seems much more probable.)