The required procedure is therefore to identify who is wishing to enter the cockpit using the viewer on the door. Can the operating pilot do that from their seat?
Your second to last paragraph - if you think what you state, how you know the person sat next to you is any better? Wasn't there a recent hi-jack by a F/O when left alone in the cockpit?
You've missed the point yet again. A bouncer from Mothercare will not be able to stop me doing from doing what I want with the aircraft. But anybody big enough potentially becomes an even greater risk because I can hear it now; the mouth-breathing knuckle dragger walks into the cabin - "I fort dey wer about to take-over de plane. So I wacked 'em. Can anybodee fly wun of dese fings?"
Get real. Think of the problem and then the solution. Muppets in the flight deck won't work. And we also have to balance to the solution with the threat. This sort of behaviour is spectacular, but fortunately very rare. And we do not want to suffer from an ill considered over-reaction. So treat this as an engine warning in flight. So first you fly the plane and then...