That's just it: You can't possibly know how many potential hijackings were never even attempted thanks to the change, but you can assume that there would be copycat attacks if nothing changed. Even so, there have been multiple attempts to storm cockpits since doors were beefed up. Not sure why you think they don't happen:
http://www.sfgate.com/crime/article/Attempted-S-F-American-Airlines-cockpit-break-in-2372003.php
Definitely NOT "speculative."
he was quickly subdued, no armored door needed
even if he had briefly breached the cockpit, he would have been quickly dragged right back out
these armored doors are designed to withstand a sustained, determined assault, which is frankly stupid
no hijacker is going to be given that much time
the only time the doors will be subjected to such an assault is in situations like this where the legitimate crew have been locked out and need back in
you know, situations where an armored door is COUNTERPRODUCTIVE

