Ceramic and plastic knives exist and could easily be smuggled aboard an airplane. I think the key insight is that most people don't want to do mass harm and instead just want to live in peaceful freedom and do their own thing.
now the pilots have learned that if there's a person with a knife aboard the plane, they won't open the cockpit doors to avoid a potential hijacking and more casualties. And passengers know that if hijackers take control of the plane, the hijacker might crash it somewhere instead of holding the passengers and crew hostage, like it was done with plane hijackings before 9/11 and might fight back.
So with the example of the 9/11 attacks, the situation has changed enough that a plane hijacking with a knife is much more unlikely
That really has nothing to do with being on airplane though, does it? There are endless places where you can get within stabbing distance of a group of people.
I'd actually expect a plane to the one of the absolute safest places, because like others said, everyone on that place has just become a victim. It doesn't even matter if you like the person being victimized, you're gonna exploit that distraction and beat the fuck out of the dude.
In public people are much quicker to just run away and keep themselves safe.
