So what level of screening is acceptable?

I'm willing to say that I will accept a lower level of security, fully acknowledging that it could very well lead to a situation where the plane lands with no one left alive but the pilots. I'm ok with that. I'm ok with saying, "WTMD is fine", knowing that ceramic blades with plastic handles could get on board and be used to kill several/all passengers. As long as they don't breach the cockpit then it's an acceptable risk.

What level of "incident" would you be willing to get up in front of the families of the victims and say: "We're sorry that this happened, but this is the sort of risk we were prepared to accept?"

Because WHEN something happens, not IF, folks will be wondering "how the system failed". No one seems to be willing to say: "The system did not fail." You can't tell me that hijacked planes being used as weapons were not an anticipated thread - Tom Clancy had it in one of his books. But no one was willing to say: "This was a risk we were willing to accept. And it happened. Bummer."

No one is ever willing to put their money where their mouth is. When something happens, be it a car accident, a crime, or a terrorist incident, no one ever says "This was a known risk, the person who died chose that risk when they decided to do <whatever>." Instead we get screaming, crying, and more stupid laws for things like "gun free zones", shoe carnivals, and more stupid security theater.

No check will. So we treat everyone the same. That's fine, but if you want to lower the level of screening on EVERYONE and not try to apply any sort of analysis towards threat identification then you're going to, by definition, let more through. I'm willing to say that in order to do that we, as a country, have to say that "Yes, we realize this has the following risks, and we're good with that" like when your doctor gives you a consent form before surgery. The problem is that we never ever seem to be willing to do that.