Remember, one plane did. After hearing about what happened to the other three planes, the passengers realized what was going on, and (almost) took back the plane. This was all on the spur of the moment, yet they rose to the occasion. If they had known what was going on slightly earlier than they did, they could have stopped the terrorists getting into the cockpit in the first place, and then they would have saved their plane as well.
Exactly, that fourth plane already proved the case, no idea why we're even having this debate. No plane will ever be successfully hijacked in the US again (or at least not for many generations), even if the hijackers have guns, because everyone now knows the plan is to crash it into something. The entire passenger compartment will bum rush the hijackers. Plus, armed Air Marshals.
The passengers of the fourth plane knew that other planes had been hijacked and crashed that same day. They made the assumption that these multiple hijackings in such a short period of time were all related and concluded that their plane would likely crash.
The probability that a random hijacking would be done with the intent of crashing the plane is far lower in general than on a day where you know another 3 hijackings did result in that. Especially when you consider how unlikely simultaneous hijackings are.
The plane only went down b/c the terrorists had taken the cockpit. They crashed it intentionally when they realized the passengers were about to retake it. With barricaded, reinforced cockpits that won't happen again, and passengers will also fight back instantly, won't give hijackers time to get control and get organized.
That the arrest was not followed up by a FISA court order allowing more investigation of the plans of the arrested person was a major screw-up on the part of the FBI.
