Fen boy shows that complete separation is not desirable in several scenarios.
The before mentioned 'cevlar-reinforced-super-titanium door' could be a compromise.
However in incapacitation scenarios cabin crew may need to enter.
So they would probably have a key.
And the aggressor could probably obtain that key.
So does it really matter whether our present door is wide-open or locked ?
With the ability of today's aggressor it seems not.
Armed guards is a possibility.
A 'quick draw' armed cockpit crewmember might also hold them back.
Prevention is better, but might be an utopia.
To start the discussion is probably a good idea.