It is an easy subject, all we have to decide is “ is my or another’s mental health in an acceptable range ? “
It is not an easy subject at all.
You can't do that decision with the necessary reliability and correctness, that is the whole issue, and that is why there is realistically not much to do.
Of course people will argue that "even if it saves only one life, it is worth it". But I strongly disagree with that. There is no zero risk life and no zero risk society, and if someone is not prepared to face the extremely low risk of ending in a suicide crash, he can seek other modes of transport. The ramifications of what you advocate on absolutely healthy and spotless crews would be totalitarian.
Aircrew are already the most selected, supervised, checked and tested professional group in the world, but as with everything, there is no 100% in life, never, nowhere. Of course you can seek to improve, but as the Germanwings crash has shown, realistically there was not a lot to do. You can't look into someones brain. That is just how it is. No crew want's to fly with someone suicidal, but it was obviously not possible to spot it in the cases that ended up in a suicide crash.
And by the way: getting rid of the locked cockpit door would immediately bring back the terrorists.