You're right that a guy with a knife might be able to bring down the entire plane, but that's only one of many ways to mass-murder people. Security lines at the airport, political rallies, crowds outside concerts and sporting events, and other concentrations of people are easy targets for an unsophisticated suicide attacker under the direction of a bomb-maker. You can't stop that kind of attack. Planes are special because (we now know) they can be extremely effective weapons in themselves, not because bringing down a plane is an effective way to kill the people on it.
Ironically, reinforced cockpit doors are a necessary part of any terrorist's plan post 9-11, because that's the only way they can protect themselves from the passengers. I think there are now protocols to take control of the plane remotely in case an attacker infiltrates the cockpit and barricades himself inside. Unfortunately, trying to search the web for that information only gets me conspiracy pages about how the government remote-controlled the planes in the 9/11 attacks. Information pollution FTW :-/
Ironically, reinforced cockpit doors are a necessary part of any terrorist's plan post 9-11, because that's the only way they can protect themselves from the passengers. I think there are now protocols to take control of the plane remotely in case an attacker infiltrates the cockpit and barricades himself inside. Unfortunately, trying to search the web for that information only gets me conspiracy pages about how the government remote-controlled the planes in the 9/11 attacks. Information pollution FTW :-/