avalibility rates vary based on national service factors like age of airframes, spare parts, training of repair crews and pilots.
350+ F35 is a world wide number so trying to use that as your basis of Avalibility is impossible. As you will have brand new demo aircraft in the hands of air forces that song yet have the infistucture to maintain them.
You will have users who have a higher Avalibility rate and some with no Avalibility rate.
Much like how an available rate for an older smaller F16A+B fleet will be less than a larger newer one.
The one that matters in this case is the Japanese.
This F35A was a Japanese F35A. It was assembled in Japan by Mitsubishi Heavy industries. Who just declared IOC a few months back and had 13 units until this loss.
I repeat what I have stated and asked all along, it is to early to jump to conclusions of cause or place blame or proclaim that this was the result of a aircraft.