How realistic do you think a shoot-down scenario will be? Take the time-scale into account for verification of hijack, intercept, confirmation of “illegal” flight status, etc. Then order the USAF/ANG aircraft to engage, taking chain of command decisions into account.
As far as time goes, the F-Teens are up flying CAP quite often. This shortens the response time to minutes.
I've had to stay at FL190 on my way up a lot longer than usual on many occasions since 9/11, due to our future F.O.s in the F-Teens. It doesn't happen as often any more, but for a while there, I had several escorts that I could see, every time I departed BWI!
Knowing that I was being tracked by an armed fighter did NOT give me a warm and fuzzy feeling! What would happen if he accidentally shot me down? I guess that it would be preferable to simply arming pilots........