At some point a risk/benefit assessment needs to be done.
Obviously we cannot secure ourselves against EVERY eventuality. No, no, no, we cannot. It's not possible.
Somewhere, in the near future, a person - maybe not even a terrorist - is going to smuggle something into a plane. A gun, a knife, something. Then there's going to be mucho media whining about HOW THAT WAS ALLOWED TO HAPPEN.
Somewhere, eventually, someone will commit a terrorist attack on a plane. Maybe they will do it by smuggling weaponry on board. Maybe they will place an anti-aircraft landmine [these DO exist] near the runway. Maybe they will do a million other things. Maybe we will get lucky on that day and only one or two people will die. Maybe hundreds. I do not know.
Somewhere, one of two things will need to happen:
Either people - or a subgroup thereof - will put their foot down and decide that they'd rather live with, say, 1% annualized chance of a terrorist attack than sacrifice more liberty and property and prosperity to try and bring it down further.
Or the surveillance state will expand forever, and the terrorists would have won.