"Never again will a terrorist be able to breach the cockpit simply with a box cutter or a knife. The cockpit doors have been reinforced, and passengers, flight crews and air marshals would intervene."

Not true. Cockpit door opens when the captain takes a piss. Flight crews are usually sleeping at the back of the plane or stuck behind those beverage carts. Very few flights have air marshals and most passangers are tied up in their chairs with the seatbelt. I agree it is less likely than in the past, but definitely not impossible.