You're forgetting that quite a number of DC-10s are also still in cargo and passenger service.
The DC-3 has been around for nearly 70 years. Douglas designed it to last forever. With proper maintenance and an O2
tank, they'll be flying them on Mars.
The fact is, there is no real limit to how long an airframe will last if it is properly maintained.
Those who fail to learn history are doomed to repeat it in summer school.