This is the part that's the problem. From the evidence available to me,
there is no simple/fixed relationship between hours flown and maintenance
costs (even excluding fixed items like annuals).


You are absolutely correct, Sir!

This has got to be one of the most irritating things there is about managing
a plane. If it flies too little, it costs more. If it flies too much, it
costs more.

I suppose one thing is true - it will cost more no matter what!