NORAD was never advertised as being able to prevent hijackings.
Throughout most of my life, there have been thousands of nuclear weapons aimed at the U.S. Some of those nuclear weapons were targeted straight at the SAC bases on which I and my family lived during both the 1962 DEFCON 2 and the 1973 DEFCON 3. When I was a kid and the network signal went out on the TV, I quickly switched channels to another network, to test whether New York had been vaporized and I thus had only a couple minutes to live. And yet, none of those nuclear weapons ever hit the U.S., or were ever used to coerce us.
So yeah, I’d say NORAD lived up to its advertising.
As for the gap in our defenses revealed by Mohamed Atta on the morning of 9/11: that problem was fixed within an hour by a small group of heroes in the skies above Pennsylvania. Their number included Tom Burnett, Mark Bingham, Jeremy Glick, Todd Beamer, and Sandra Bradshaw.