Sadly, a locked cockpit door is the final obstacle that anyone determined to endanger the flight crew has to encounter. When that happens it means that all other security measures have failed, which is unacceptable. There are too many gaping holes in the security net we have at the moment, even before someone with criminal intent boards an aircraft. Look at the news.
Notwithstanding the above, a steel, lockable, cockpit door will not prevent a determined terrorist from blowing up the aircraft or taking control of it by many engenious, determined people. Once they get to the aircraft threshold, you are probably damned. Little comfort when you have lost control of the aircraft that you have even a 6 inch armour palted door covering your back!
Pass the buck to the airline and hold them accountable! That's what the authorities do, whilst they ignore very efficient security detterents available on the market that could very significantly reduce the threat of a terrorist getting past check-in! But that costs money, ha!