Original Message

Correctly maintained machines can fly for many decades. Now then, once parts become too expensive or manufactures add too many maintenance requirements for a company to make money then the machine is not worth continuing with.
Most operators don’t have the luxury of unlimited money like the US government to buy new aircraft whenever the perceived need arises.
My observation of some pilots around me is they are used to new machines every few years as that is what they were used to in the military.

For the owners it’s all cost vs. gain and pilot or crew desires come in far down the list.