True, consistent fuel efficiency is a huge part of the limited design life of modern aircraft, and why it makes sense for airliners to pay for brand new aircraft to replace perfectly adequate 10 year old aircraft on competitive routes. And the fuel economy of the Comet and 707 was awful. But to my knowledge the DC-3 fuel economy doesn't compare favourably on a passenger-km or tonne km basis with later generation jet aircraft that have been retired

Difference is you can keep an ancient DC-3 around for occasional use with relatively basic maintenance, whereas a second generation jet or turboprop comes with service and life-limited parts requirements to make it considered airworthy that would cost more than the aircraft is worth...