Obviously they could not, for two reasons. One the one hand, the cockpit doors are locked, hopefully for good, and on the other hand, the security protocols that existed prior to September 11 allowed knives on board. If I remember correctly, they were allowed to have knives up to nine centimetres long. The reason for it being nine centimetres was that up to that point you could still stab somebody without killing him. This was the assumption then. This assumption was revoked shortly after that, but those two major risks were mitigated by the existing system.
The question is, what are the risks today and are the systems mitigating all the risks that you perceive today?