When you subject aluminum to cyclic loadings, there is a relationship of cycles it takes to crack/ vs material thickness, or structural meat you might say. If the structure is sufficiently meaty, the number of load cycles to generate cracks become effectively infinite.
Modern airliners have structures carefully designed by analysis to last a certain number of load cycles, then subjected to a fatigue test in a rig to simulate the loads, then whatever cracks turn up have to happen at at least 3 times the number of cycles you are designing for. Put another way, if a crack shows up at cycle X, the life of the structure is set at 1/3 of X.
Airplanes like the DC3 were designed before all this was well understood, and in the slide rule era, so they added lots of meat to be safe, and the result is the 3 has a fatigue life of more or less infinity.
What brings them down, besides accidents, is corrosion in its various forms. Deal with that and a DC3 will theoretically run forever.
In my book the '3 is the world's greatest engineered transportation device. I can't think of any other machine that, 85 years after it was designed and 74 years after it was last built, businesses still, in 2019, buy these ancient airframes not to fly around as toys or museum peices, but to put them to work to make money.