Good points being made.
Only thing I would add is the inherent 'overengineering' apparent in some older designs like the DC-9. Without CADCAM etc the engineers erred on the cautious side resulting in airframes that could easily last half a century or more. As long as fuel stays cheap they're still viable.
While working throughout in Venezuela in the 90s, the main and regional airports were all full of Avensa and Servivensa DC-9s (many still with their ex-Eastern livery colours still showing through several bad paint jobs). Quite a lot of DC-3s too, but that's another story.