Sounds obvious but depreciation is most dependent on purchase price. If you get a bargain it won't depreciate much, I've made money of quality lenses I have bought after keeping them for years. As above the older lenses cannot be serviced but they have also nearly hit their bottom price and the pre IS and VR lenses from Canon and Nikon are still fantastic performers. The older lenses may not give the full FPS on mirrorless cameras so that may knock a bit from their value in years to come.