A practical, objective standard, approximating the way we evaluate the performance of other LEO's. [law enforcement officers] *Im sure that there are many - some you may like more than others!
An objective $cost : benefit ratio.
Your chronology is correct. Sky Marshals started in ~'61, the attacks of 09/11/01 in the following millenniumexpensive compared to what? the twin towers? a plane crashing into the pentagon?
So that's 40 years of doodly squat, and the sky marshal program didn't even slow the attacks of 09/11/01 down a little. BUT !!
If you recall, 4 planes skyjacked, by 19 mostly Saudi Middle Easterners.
I'd read bin Laden originally planned 10 different planes be skyjacked. Evidently he couldn't find more than 19 suicidal terrorists, so settled for 3 teams of 5, and one team of 4.
Would have been nice if Beemer's bonkers could have retaken the plane and brought 'er down gentle.
* A year or two ago I submitted a written incident report to a New York State Trooper. 5 pages (it was a summary of a 5+ year crime spree). She read it in my presence, and noted in the report "L.E.O.", and she said aloud, -yea, I know, Law Enforcement Officer-. Before I could censor myself I blurted out, "It's also Low Earth Orbit." She liked me even less after I said that.