All the armoured cockpit door will do is prevent the terrorists from crashing the aircraft into their target of choice.
What do you think the terrorists are going to do when they find they cannot get into the cockpit, return to their seats and have a nice cool glass of milk while you take them to the nearest FBI unit?
Get real. When they cannot take control of the aircraft their second choice will be to cause it to crash rather than be taken alive. You and your passengers will be just as dead as if you had gatecrashed a party at the White House. In my opinion, the firearm on the flight deck is there for just this situation. When all else fails and everyone on the aircraft is going to die in any case losing a few passengers to stray rounds to save the rest, though tragic, is sensible and justified. (After all, the government thinks losing everybody to a fighter is justified to save their back-sides).