I find your certainty about something you appear to know nothing about mind boggling. Every crash is a deficiency of sorts, so let’s just tell Airbus to build planes that don’t crash! Easy! Let’s just have peace instead of war! Obvious!
It is not sufficient to point out “obvious deficiencies”, one must also point out possible improvements. As other posters (with more patience than I can muster) have pointed out, it is not obvious how to improve this. Putting a lock in place to prevent movement of the lever adds another level of complexity that can itself fail. This is well known and has been extensively studied (a classic paper is No good deed goes unpunished: Case studies of incidents and potential incidents caused by protective systems [0]).
Listen, for example, to this episode of “Undercover Economist” Tim Hartford’s Cautionary Tales [1] (though the example given might itself be problematic [2]).
Airplanes are extraordinarily safe already (look at the amazingly low accident rate, given those fallible, sometimes even negligent and incompetent people that fly them).
Contrary to your facile assertion “There is lots of room for improvement”, there are very few obvious improvements. Change something and you might prevent one accident but enable another one.
As an example: terrorists enter the cockpit. Easy, obvious: lock the door! Ok. Suicidal pilot locks out the other pilot and flies into a mountain. Easy, obvious: unlock the door. GOTO 10. It’s not obvious. These are hard problems that many competent people have thought about in depth. That’s why “everyone” is so “intent on defending the status-quo”.
In other areas, there might be “obvious improvements”, eg regulatory matters, but even there it’s not easy. Aviation is operating within an international framework, there are rules and international treaties and regulatory bodies and stakeholders and so on.
But my main point is that it’s absurd to accuse Airbus of overlooking an obvious improvement without even a cursory understanding of the matter.
[0] https://aiche.onlinelibrary.wiley.com/doi/abs/10.1002/prs.68...
[1] https://timharford.com/2019/11/cautionary-tales-ep-3-lala-la...
[2] https://leancrew.com/all-this/2019/11/galileo-and-failure/