Cui bono?
Saudi Arabia is increasing production, breaking the OPEC limits. Others will do likewise in response. Oil futures are down since the attacks. (We've had no crude supply problems for some time -- the shortages, to the extent they've been real, have been the result of refining capacity shortfalls.)
Maybe it was a foregone conclusion that something like this would eventually happen and rather than focussing attention and resources to preventing it they were focussed on how to respond in the aftermath (domestically, at least.)
I don't doubt for a minute that it was a forgone conclusion, but I fundamentally disagree that the passive approach was the right one. And I do not believe that anyone with the interests of the U.S. imperium in mind, would have traded an increased flow in oil production for the damage wrought to our economy by the 9/11 attacks.