The biggest thing that stopped 9/11 attacks since 9/11 was locks on cockpit doors.
The second biggest thing that stopped them since 9/11 is passenger awareness that they could just... Not let a few assholes with boxcutters fly a plane into a building.
After 9/11 people are not going to let hijackers run the plane. They may hurt you in retaking the plane, but if they fly it into a building you'll be dead for sure.
