Aeroplanes are only as old as they feel. With care, regular check ups and attention to detail they will last indefinitely. The problem is that the cost of care rises with the passage of time.
My father has an excellent hammer that he got from his father. Its as good as the day it was bought new from the hardware store and its only had two new heads and six new handles in its seventy years of service. On the other hand, I scrapped an axe last week when I found that a new one-piece all-steel axe costs less than the price of a new wooden handle. As tinyrice said, its all a question of economics. Try buying a new Merlin for your Spitfire and you'll soon see what I mean..
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Through difficulties to the cinema