That people with actual mental problems have been there, and do get there, points to a huge cultural and systematic failure of the industry.
There are no easy solutions, and the good ones will cost money; but companies have to assume full responsibility for who they hire and place at the controls of their aircraft, for every flight.
Such cultures must care for the development and wellbeing of their flight crews, including providing the means to help where necessary and safety nets in case problems cannot be rectified.
It goes without saying that cultures built on financial exploitation of pilots such as P2F or zero hours contracts, competing in a race to the bottom, must not have a future.
Spot on. There will have to be a much greater focus on regular checking of mental health. This will come at a financial cost to pax and a cost of greater intrusiveness into the 'private lives' of crew, but it needs to be done.
We also need to 'normalise' poor mental health in our culture. Depression is a default human reaction to trauma (e.g. bereavement, relationship breakdown, suffering a life-threatening event) and millions of us will suffer it, although the overwhelming majority of us recover. It's very common and not a reflection on the individual. Employers need to put in place care and reassurance programs to deal with it. An employee ringing in with a sick note with a diagnosis of depression shouldn't feel more scared or ashamed than one ringing in with a sick note for flu.
(I would just add that IMHO this pilot was psychotic, not depressed.)