You contradict yourself. You claim that the new cockpit doors have "only caused harm", then point out zero attempts to seize an airplane since the change.
No contradiction here. Zero attempts doesn't mean the doors didn't cause harm. I'm thinking of the Germanwings suicide.
More important than a likely difference in passenger response is the far greater difficulty in getting into the cockpit, no matter how the passengers respond. Terrorist could still crash a plane, if they got into the cockpit, before the passengers knew what was happening. Also, remember that passenger rebellion on flight 93 still resulted in a crash and everyone on the plane dying.
Because they retook the plane rather than keeping them out of the cockpit in the first place.
Plus knowledge of an potential armed air marshal also deters hijackers armed with less than guns.
The number of flights with air marshals is low. Furthermore, the air marshals aren't that well hidden, a hijacker very well might see them as a source of a gun.
Passenger revolt would help minimize harm in a hijacking without guns, but be far less effective if they had guns,
It would still work even with guns, although more would die in the process. Yes, charging a terrorist with a gun comes close to being a suicide mission--but at this point we know that letting them control the plane (as opposed to simply giving orders--note that guy who hijacked the Egyptian plane but didn't try to enter the cockpit. He wasn't stomped on) is suicide anyway. If you charge you give your loved ones on the plane a chance they wouldn't have otherwise.
Regardless, the point is that it was not knives but planes that were the key weapons used in 9/11, and guns would have made their task far easier, prevented the revolt on flight 93, and would make a repeat of 9/11 far more likely. Two unskilled schmucks with AK-47s could mow down an entire plane full of 300 passengers. The passengers path to try and get to the gunmen would be blocked by the first rows of dead passengers killed in seconds. In contrast, 2 ninjas with box cutters couldn't kill more than several people before being overrun. The point is that Trautsi's claim that assault rifles and box cutters are equivalent and thus should be treated as such by the law is absurd.
Two schmucks with AK-47s could kill more passengers but they wouldn't get the plane--and they wouldn't fare very well when the pilots demonstrated where the actual maneuvering limits of a jetliner are. Not to mention passing out from a lack of oxygen.
