@321. It is simple economics: if you buy an old airframe cheap and fly it a little at higher DOC's it might still be cheaper than a new more efficient airframe. The best part: you can park it a few hours a day or a few months a year without to much pain. So as long as you maintain it well there might not be any reason to go for new unless production goes way up year round then the business case for a more efficient but more expensive to buy/ lease airframe becomes viable...
So i think the engineering department is almost leading in deciding when to replace an airframe for a younger one.. they are getting the bills
