Better cockpit doors had been suggested for many years. Hijackings happened all the time. Cmon.
US hijackings pretty much ended in the 1970’s when we adapted in airport screenings. Also, hijacking training for air crews called for cooperation with the hijackers. Unfortunately, the training didn’t work in 2001.
It was not lack of money that allowed 9-11, it was the lack of imagination of what evil people would actually do.