Not trying to be scientific here. I was simply making a common phrase in speaking generalities about steel. However, with my observations from my experiences with steel, it may not have a scheduled time of it's engineered design's demise, but time is not on steels side. Many variables can expedite it's demise and in the environment of the saxophone it is placed within an area that is not always so kind to steel. Water, aka saliva, even worse possibly, increases its slow process of no longer being steel.What "simple physics" is that?
F=-kx has no time dependency.
I have a background of classic car restoration and it has always been my observation that old springs no longer hold the tension they originally were designed to hold. A common term used for steel when it is fatigued in an automobile spring is 'sag'.