I had a similar opinion of myself, in that I deluded myself that I was indispensable and irreplaceable and having accrued a large EOD and CIED database in my head, truly felt that 'going sick' would let everyone down.
Regardless of PR campaigns, there remains a stigma attached to mental health issues in the British Military, and I assume this is common in other cliquey professions. I needed a couple of near-misses for me to seek help, and it was actually of great relief that I could stop pretending.
Whilst at the time my future looked bleak, as I felt my value was entirely my experience and the contents of my head, I have since had assistance in raising my 'share-price' in other areas, so there ARE alternative if you look hard enough.
We actually know not much about Mr Lubitz, some details that point to explanations, but if he was secretly on medication, these can have unpleasant side effects, especially if he sourced them by dubious means.
I must say your post has been very enlightening.
I think it is ignorant and unfair to stigmatise mental health issues. Like a close friend of mine with whom I do some family law case would always say to me on receipt of yet another appalling and soul-destroying psych report "Ain't we all crazy?". We are human and subject to emotional difficulties - how we handle it and treat others who may have difficulties is what makes the difference. I am pretty sure if this young man felt he could still have his job despite his mental health issues, he'd probably have been more open with his difficulties and received better support and perhaps avert this tragedy. Like they say, a problem shared is a problem half solved. Having to hide mental health difficulties or any difficulties for that matter only further compounds the underlying problem! On the flip side, what is the aviation industry or any other industry to do? If someone is truly unwell to the point of incapacitation whether physical or mental that would impact on their ability to do their job, the question would be, like you rightly pointed out - "Am I that indispensable, irreplaceable?" The reality is, no one is. The world will go on with or without us - and I learnt that the hard way, as I progressed through my career.
The added pressure brought on Lubitz from his perceived need to hide his issues from his employers could have been an added catalyst. I see this a lot in court proceedings where good, normal parents come into litigation regarding their children and a few years or months down the line having being subjected to unjust scrutiny, innumerable assessments, and unbelievable violation of their human rights, we start to see these hitherto perfectly normal folks break down and disintegrate and have mental health issues that they never had before they were shoved into the pressure cooker of the family court. In these fragile states, these parents will either have to learn to hide their anxiety disorders, depression, PTSD etc which are brought on by constant harassment from Social Services etc - and this includes hiding these symptoms from their doctors which means no treatment or medication is ever prescribed and therefore no evidence of mental health issue is ever recorded in order not to lose custody of their children or tell the truth to the family court and they will immediately be punished by the permanent removal of their children for adoption! In the end, if the litigation is protracted, something always, always gives! We have as a result had very avoidable tragic outcomes where murder/suicide involving infanticide becomes the outcome in a handful of these cases. If the approach was less punitive and fairer, the outcome is always positive. I have had numerous uphill battles where I successfully argued in order to differentiate between psychological/emotional difficulties brought on as a result of vexatious/malicious litigation and underlying serious psychiatric conditions that may undermine good parenting. In each case, the problems are magnified with the inability to openly share one's difficulties fearing that what one loves the most (in this case children) would be removed as a result of this difficulty. So this problem is not restricted to professions only, it spans through the various spheres of human life, societal expectations/misgivings and it all boils down to lack of knowledge/understanding of mental illnesses. It is a fine line and a balancing act but I hope some lessons can be learnt from this tragedy.