That certainly would have been true in the old days, but I dont know about now. Today, 10,000 parents surround Columbine listening to their kids get murdered. It would not be hard to believe that only the Pennsylvania plane had a pilot that refused, thus the crash that did not go inot a building.
If the Pennsylvania plane did not succeed, then somehting different happened on that plane, like resisting. If the Pennsylvania plane did not have a terrorist to fly it, then none of them did, and the other 3 pilots did what they were told.
I think only 1 of the 4 pilots resisted, and forced crashed his plane.
No terrorist is going to let the success of his plan rest with someone else.
In any event it is very hard to make that final dive on target as the Japanese found during WW II.