>> nearly 40 years is a very long time in airplane years
The reason it isn't, in fact, a very long time is that airplanes, unlike most other vehicles, are subject to highly rigorous regular maintenance and part replacement programs. The airframes themselves can last essentially forever (well beyond the decades time frame that we're discussing) if specified maintenance continues to be performed. Witness the many WWII and earlier aircraft still flying at seven or more decades of life.
The reason it isn't, in fact, a very long time is that airplanes, unlike most other vehicles, are subject to highly rigorous regular maintenance and part replacement programs. The airframes themselves can last essentially forever (well beyond the decades time frame that we're discussing) if specified maintenance continues to be performed. Witness the many WWII and earlier aircraft still flying at seven or more decades of life.