The level of stress induced in modern coil springs is supposed to be designed to be below the plastic limit, i.e. the point at which the metal will undergo plastic yield and therefore take a set. Repeated stressing at close to the yield point can cause minimal deformation. Enough high stress cycles can cause fatigue, and fracture. Spring steel is selected to have a very high resistance to fatigue, on the order usually of several million cycles. The reason 747 wings, which routinely flap 5' at the tips, don't fall off, is because engineers know how to design for fatigue.
The level of stress induced in many old flat springs was much higher, and the failure rates were too.
Unfortunately, back many years ago, someone also invented "value engineering". Which says real basically, is the increased costs worth it, or are the savings worth the risk. So it is not worth the risk, monetary liability wise, for Boeing's engineers to buy cheaper metal to make the wings out of on a 747, but it was very probably judged worth the risk to buy gun springs that would only last 15 to 60 years in the 'normal' range of use.
What could have happened... did.