Not true that a duty-free bottle has not been used in an attempted hijacking. Last year in South america (Brazil?) a looney who had replaced the whiskey in a bottle with petrol poured it over a couple of seat backs and threatened to set it alight. He was taken down by the cabin crew.
Attempts have also been made using just fists (Algeria?) asthma inhaler, chopsticks and even a tv remote control (that one was successful) just in the last couple of years. We used to let the hijacker succeed, in order to prevent loss of life, but now we know that some of them are suicidal and cannot be allowed to take control of the airplane.
But no matter what the hijacker is armed with, crew and pax can use the seat cushions, which usually have straps on the back to enable them to be held like a shield (meant to be used as flotation devices). If a group goes for the hijacker(s) they should be able to get them down even if the crims are armed with knives, bottles or whatever.