AIUI, the cockpit doors are now relatively more secure, and there are significantly (i.e., 2 orders of magnitude) more federal air marshals. Not to mention the changed calculus for the passengers if a hijacking were to occur.
If there were no air marshals, and the cockpit door was replaced with a 1970s-style beaded curtain hippie door, I would still expect future hijacking attempts to fail.
Before 2001, a hijacking was almost guaranteed to be an inconvenience for the rest of the passengers. A diversion to another airport, being stuck on the plane while negotiations happen, etc. After 2001 every passenger is going to assume they're being commandeered as a missile. There will be tons of resistance from 1A all the way to 33F.
(Strong cockpit doors are still a good idea. I'm not arguing otherwise. But the changed calculus you mention is enormous. It alone I think would make successful hijackings a rare occurrence.)
I agree though. Anyone familiar with 9/11 would fight hijackers with their life.
There's not going to be a common hijacking scenario where the passengers have to get into the cockpit.
When the hijacker is the co-pilot
Hopefully the resistance begins before a hijacker has a chance to jump over the beverage cart while one of the pilots is going to take a leak.