I shoot Leica and have bought and sold a few lenses. I have always profited, even if I paid "normal" used price when buying. I know at least one lens I owned that was worth $1100 in 2006 is now going for well over $2000. So yeah, if you choose lenses carefully, they will generally all appreciate. Bodies, not so much.
And I can't think of a time when Leica lens values went down rather than up.