"but post 9/11 the chances of this happening have gone waaaay down"
How do you as SLF know this? Why are thousands airline pilots who intimately know the security procedures post 9/11 still worried about it? I promise you carrying a weapon is a pain in the butt, I get no thrill from it. Its done because of a previously manifested threat to security of the aircraft and the belief that it will at least be tried again.
"And I don't see concentrations of ME-ancestry young men sitting together in business class any more"
There is nothing to stop them from doing so if that's how they arrange to buy the tickets. Don't be profiling now, not allowed.
It comes down to your belief that you think the SLF will in mass rush an attacker with a gun or other weapon. I don't have the same degree of trust in human nature, especially when faced with extreme threat that they will do as you have predicted. I've seen people under stress in tenious situations while in the military, they don't always act as predicted.
"Your average SLF with his wife and kids aboard will not be sitting on his hands"
Again, can you absolutely predict you will be the third guy to rush the hijacker with the gun after he has killed the two ahead of you who tried? What if God forbid he has your wife or one of your children and has a gun to their head screaming for everyone to sit back or else. You may then be the one who tries to stop any do gooders as it may harm a loved one. A million different scenarios and as many differing human reactions.