Against a bunch of terrorists with machine guns?
You have to round up passengers one at a time...and in the achille laura they put all the passengers in the lounge. except for Klinghoffer, everyone obeyed the shitheads...but nowadays, people might not "obey" because unlike back then, where you had a chance, they might figure they didn't have a chance...and rebel. Yes, if you line up in front of machine guns, you don't have a chance...but there are a limited number of clips in the guns carried by terrorists--they wouldn't have machine guns but automatic rifles, and so they couldn't kill everyone...
And like the Pennsylvania flight, even if a lot of passengers got killed, the story of a bunch of US or UK civilians fighting back would be a propaganda disaster for terrorists, who rely on the passivity of the west for their success...