Judging by how much the very first Creator Expert buildings are going for now, I'd say desirable sets keep appreciating in value. Looking at it another way, there will be getting rarer over time as MISB boxed sets change hands and some get opened and built. If you think the Wall-E set is fetching silly money now, wait another five years and see what happens. I really wish I bought another one when it was still in circulation. The one I have is built (but now disassembled and put back into its box. Yet, it is worth far more used than when new.
I'm tempted to sell some of my sets that I don't think will appreciate for a long time e.g Creator Expert Tower Bridge, which was in production for far too long.