What is there to debate? Even if Al Qaeda had managed to crash the fourth plane into the a White House as planned, Bush wasn't even there. He was in Florida. Even if Bush and Cheney had been in the White House during the attack, that still would've left the Speaker of the House. Continuity of government, man. Personally, I would argue that Al Qaeda succeeded beyond their wildest dreams, provoking a huge overreaction from the United States and managing to get us to do a number of dumb things while compromising our alleged core values. In response to 9/11 and in no particular order we: authorized unlimited, never ending warfare, indefinite detention, torture, mass surveillance of our own citizens and the unprovoked invasion of Iraq (which destabilized an entire region of the world and eventually led to the rise of ISIS). Somehow, I don't think this is what you had in mind, though. Again, a terrorist organization is a threat to our civilians, but they're not going to cause the collapse of the United States. A few thousand men with machine guns and a forth grade education aren't capable of it. Relax.