even if such a knife got past security, anyone who tried to take over a plane using it would immediately get jumped by - probably all - the passengers
You know, I've heard this line of reasoning a couple of times and I'm not quite sure it holds up. I think that most people in that sort of situation wouldn't want to be cut with a pocket knife regardless of the implied threat of the guy crashing the plane into a building. Some people would probably attack the guy with the knife, but I don't think that subset of the population would have increased drastically post-9/11.
Keep in mind that al-Qaeda sent teams of five for each of the planes they hijacked. They specifically brought extra folks on board to handle a passenger upset like a passenger tackling the first terrorist that brandished his knife. Although I've not read the 9/11 Commission report in its entirety, I would suspect that most of the hijackers remained seated to retain the element of surprise in such a situation. Given that the hijackers rehearsed their attack many times over, I doubt that a post-9/11 group of passengers would fare much better.