The 9-11 hijackers used razor blades. So, its been done. Of course, no American passenger is going to believe that everything will be fine if they comply these days so I don't think its possible to pull off 9-11 again.
Only in the days when we didn't lock the cockpit doors. Now that the cockpit doors are locked, even an actual gun wouldn't be sufficient to hijack the plane (the cockpit doors are essentially impossible to open from the outside, at least not without equipment that would be impossible to bring on a plane and would be rather conspicuously slow to use during a flight).
Post 9/11 passengers are not going to allow a guy with a razor blade hijack a plane, or even several guys with razor blades. Passengers and crew members now fight back and have the advantage of numbers.
Yeah, but what if _everyone_ on the plane were a hijacker?
Wait...