Imagine someone ploughing shortly after the start into the Super Bowl. Thats what REAL security nightmares are made of.
Even though, even that cant be fully prevented. Best example would be someone like Andreas Lubitz.
That could be the basis of the worst reality TV show ever! Or one of President Trump's economic programs. Or both.
The most important safety measure against this became mandatory procedure after 9/11: locking the cockpit doors. If the terrorists can't control the plane, they can't use it as a projectile weapon. No amount of killing passengers will give them this kind of access.
The reason why 9/11 was possible was because it had never happened. They could have added zero security measures, not even locking the cockpit door, and 9/11 still would probably not happen again. The reason is because before 9/11 the common wisdom was to let the hijackers do what they want. After 9/11 the common wisdom is the plane is going to be used to fly into a building.
But, locking the door is still a good common sense extra security measure.