Unproven? What's "unproven"? That a safety regulation always prevents what it is supposed to prevent?
If co-pilot Lubitz couldn't be alone in the cockpit, it is much more likely that he would have killed himself somewhere else - otherwise risking that he would have had to fight, and might not be successful in his suicide attempt.
Safety regulations exist to mitigate risk, but they do not eliminate risk. "Would have, could have, should have" does not work in situations like this one. There is absolutely no proof that following a different safety procedure would have prevented a man from allegedly committing suicide with a plane and killing all passengers on board. It's a tragic accident. Lufthansa may decide to implement a new safety procedure in the future, but it is no guarantee that another similar incident won't occur again.
The fact remains that you are far more likely to die in the car that is taking you to the airport than in a plane.
Nothing eliminates all risk. But, since the implementation of secure cockpit doors, post 911, how many planes have been taken over by terrorists breaking into the cockpit? None.
Suicidal pilots wishing to take a planeload of passengers with them into eternity are not exactly common. Yet making it very difficult for them to do so, after having had the experience of the first one (or probably the second, since that was likely the case with the Malaysian Airlines disappearance) at no cost to the airline should be expected. If someone were suing the airline under general principles of american law, not providing such a deterrence would be considered negligence. Probably just as negligent as flying a commercial airliner through a war zone to save a few gallons of fuel, which, as we have seen, can also kill a lot of passengers.
One is more likely to die in a car crash than an airline crash. But if one is going to fly, safer to fly on an airline that screens its baggage for bombs than one that does not. Also safer to fly on an airline that employs the latest standard procedures for cockpit security.