I've sold 6 Leica R series lenses this year and most have sold for about what I paid for them. Some up to 10 years ago. So investment? No. But they can certainly hold value better than a lot of other camera gear. Mine were used but looked after. Plays a little havoc with your depreciation schedule though :-)
Question, are you considering the purchase value of the dollar 10 years ago with today? It is not the same as it was.