Most of them have been. The death knell of the DC3 was the Vietnam War. They were used there so heavily that the stockpile of spare parts (especially engines and engine parts) left over from WW2 was so depleted that they were no longer cheap to maintain. Then fuel prices started to skyrocket, and their energy efficiency started to tell against them.
Interestingly, one of the features that contributed most to their longevity was unintended - the alloy used in their structure seems to be virtually immune to fatigue, something that was not even thought about when they were designed.
John

