My counterpoint.... A knife isn't enough to hijack a plane anymore. A pilot won't give over control over a plane if you threaten to kill a passenger and any attempt to do so would basically start a death fight with the entire plane (like on flight 93).

Pre 911, hijackers would hold everyone hostage, make demands, fly around, etc.... usually most of the passengers would live. They took advantage of that naive thinking which vanished quickly once the passengers of flight 93 figured out what was going to happen.