I appreciate the time you have taken to respond but I disagree with nearly all your objections. The central fact is that implementation of the "two persons" principle makes it far more difficult, by orders of magnitude, for a Lubitz inclined pilot to repeat that specific method of self-destruction.
The recent disclosures that Lubitz had been researching suicide and the door lock mechanism means he planned this in advance. If you make this *specific* method of murder-suicide harder the next Lubitz will simply choose another method. As plenty of pilots have said, there are many options to bring down an airliner as PF.
So changing the rule to "two in cockpit" doesn't prevent the next Lubitz but does open up the possibility of a rogue CC having only one pilot to take out, instead of two. It doesn't seem to be a well considered change.