There may be a way to increase flight deck security and allow crew to move in and out of the cabin as necessary. I understand that El Al have fitted on all their aircraft an armoured interlinked double door. There is space between the doors for one person, and one person only. The inner door cannot be opened unless the outer door is locked, rather like an airlock.
Combine this with a viewer, and it would be very difficult for a hijacker to enter the flight deck, but a pilot could still go back to check downlock viewers etc, and the cabin crew could come forward with food and the regular cups of tea that we British types cannot go more than 20 minutes without.
What do you think?