Missiles are bound to the laws of entropy just like the rest of us. Through a combination of physical conditions breaking down, like oxidation, battery degradation, and moisture, a missile will literally expire. While it may still work in some capacity, the likelihood of it not working, or simply exploding in your face is much higher. As munitions get older the support system that keeps them working does as well, and as industry shifts towards newer and better technology, keeping the old stuff going becomes harder and more expensive.We see this probably most prominently in America’s nuclear arsenal, where Minuteman III’s which were only supposed to be around for a decade are 55 years old and in active service to this day.