The crew member could open the door!
We have this policy in place at work. Doesn't bother me. Nice to have a chat with the crew whilst my colleague is out the flight deck. Also good to have someone in the flight deck to help with an incapacitation, should it happen.
I agree that should you have one person who is determined to commit an atrocity then you will not stop them with a crew member in the flight deck, but it would have prevented this tragic accident on this occasion.