The door locking logic needs re-thought I think. Nowadays there is no way passengers would just sit there whilst someone threatened crew to try and gain entry into the cockpit, so the danger of a crew member giving the code to a terrorist under violence and gaining entry to the cockpit is a bit of a red herring I think (and actually less dangerous than a terrorist managing to become a crew member in the first place). I 100% understand the reasons why that was implemented, but it was a reactive measure, and I'm not sure its foundations are as solid today as they were 10 years ago.

Given the option between possibly being killed yourself if you do something, and knowing everyone will die if you do nothing, I suspect a fairly large number of people on board would give overturning the terrorists a go.

Where the industry goes from here I don't know, but irrespective of how rare these pilot-led type of events are historically, there will need to be some kind of response, as certainly this one could have been prevented if there had been a way to get back into the cockpit.