I'm not sure this guy was insane so much as severely depressed. He did kill a plane full of people but it was a much more passive act than having to swing an ax at someone else in close quarters. Why do you think he waited for the other pilot to go to the crapper?
Also, the argument about now having to judge other cabin crew members mental state falls a little flat since the pilot that left the cockpit clearly wasn't even able to judge the guy he was with. It's sort of like arguing that now you have to read two minds instead of one.
He was seeing a number of psychiatrists, and 41 doctors total, in part because he thought he was going blind and that the blindness would destroy his career. But doctors thought that problem was psychosomatic (http://www.bloomberg.com/news/articles/2015-03-27/germanwing...).
I'm not a psychologist though, so not qualified to weigh in on questions of sanity. For the same reason, I also agree with your point about crew judging each other's mental states. The part that resonated with me was more that there's now one more person in the cockpit who could potentially pull this off.
The fact that he waited for the pilot to leave doesn't prove that he wasn't prepared to pull off the same attack if the pilot didn't leave. He'd still choose to do it this way given the option. We'll never know, but regardless, it remains a possible vector in the future, yet the cockpit security remains in place (for good reason).
Also, the argument about now having to judge other cabin crew members mental state falls a little flat since the pilot that left the cockpit clearly wasn't even able to judge the guy he was with. It's sort of like arguing that now you have to read two minds instead of one.