Hmmm....
'Locked cockpit doors = safety'
Bit of an untested assumption.
The 11 Sep. hijackers were able to take command because the policy at the time was to cooperate with hijackers. It could have been a jagged piece of plastic cup and they would have gained control. All they would have had to do was grab someone & slit the victims carotid artery as a first action. Or use a cheapie disposable pen to poke a victim's eyes out. It matters not what weapons they had, just a vivid demonstration of threat & an intent to enforce the threat.
That of course is no longer the case. Now the policy is reversed & all on board seem to be encouraged to defend the a/c.
How does locking the door or excluding known safe visitors help? Not much. America already had a locked cockpit/no visitors policy. Effective wasn't it?
No door fitted to an a/c will stop a determined attack. A bullet proof security door is great once you gain access. It WILL delay an attempt to regain control. How much time do you think a person intent on crashing needs to reach the ground as rapidly as possible?
How much easier is it is for a hijacker to gain control when there are fewer people to fight in the cockpit.
Meanwhile grannies have their knitting needles taken from them but are allowed on with their fountain pens & bottles of duty free, and pilots can't bring colleagues, family & friends to the flightdeck.