Re: A venerable workhorse
Just to add to the other options mentioned here by commenters: retired model or old airframe is a problem for airliners but not so much when one does need a special purpose aircraft. Look up "Basler Turbo BT67" - originally a DC-3 which ceased to be produced in 1950, yet the Basler conversion is still actively marketed today and is deemed very useful for some special purposes. Repurposing of readily available 74X frames with perhaps new engines may require a bit more effort than doing the same for a DC-3, but than again, 70 years from whenever Boeing decides to retire the 747 model the converters will perhaps be assisted by much more advanced technology. I wouldn't worry.