This was solved when they locked the cockpit doors. Every other airport security restriction has been an overreaction in my opinion.

Flight 11 was crashed 30 minutes after hijacking, 175 20 minutes, 77 about 45 minutes, and 93 about 35 minutes later. Even if you tighten the loop between a hijacking and getting a fighter jet in the air to counter it, it will still take you about 15 minutes at best to do it.

Then you're left with the problem of figuring out which one of the contacts on your military radar doesn't have a transponder. Go to a map like <https://flightaware.com/live/> and observe the Ohio/Pennsylvania airspace in the middle of the day. You have 15 minutes to figure out the hijacked flight, guess its flight path, and then order fighters to intercept it and shoot it down.

Quite frankly, if a plane gets hijacked in the Northeast, there is no way you can get a military jet to shoot it down in time.

Destroying an aircraft over potentially populated areas can be useful to shift who and how many people are at risk when it hits the ground, but its not really (even if you can reliably destroy it when you need to, which -- absent a highly-reliable remote destruct device built into the plane [0], is hard to guarantee) a great way to "protect those on the ground". Because the wreckage is going to hit somewhere, and you don't really have precision control of that.

[0] which raises its own problems, of course

Really, though, that was more of a glib remark. The more salient point IMO is the fact that the military is far from guaranteed to succeed in bringing down a hijacked airliner before it hits its target, or without killing other people on the ground.

Besides, since 911 the doors of plane cockpits are almost like a bank vault.

:(