Who knows?
If the two guys coming in from Kuala Lumpur are simply on a list, would you get 9/11?
If the FBI surveils these two guys - who are renting a place from a paid FBI informant - would you get 9/11?
If DC listens to FBI field people in Phoenix or Minneapolis about the funny goings on with Arabs in flight schools, would you get 9/11?
It is not all about the Wall. People made mistakes. People didn't follow through on information. There is no reason to believe that more information wouldn't made all the difference.
I agree. Who knows?! Social science isn't physics. ;)
It is just my (reasoned, I hope) opinion.