It's been said before, but I'll say it again. TWO PEOPLE ON THE FLIGHT DECK AT ALL TIMES.
Absolutely agree. Lubitz most probably wouldn't have done this if he would not have been alone. He waited until he was fully in charge and had no chance of "failure".
The question is what is a smart way of always having two people in the safe perimeter of the cockpit? To bring a flight attendant upfront creates many other issues.
I would argue the two pilots have to be the two people who always stay upfront. We need to redesign the forward space so the pilot does not have to leave the safe perimeter to go to the loo.