AAA games with a wide distribution will only go down in price, unless it is for rare collectors editions. Other games with limited runs will increase in price if they become popular (Xenoblade Chronicles). This market is actually a pretty textbook example of basic economic supply/demand theory.

As more things go digital and less discs are created, you may see the actual discs increase in value because there will always be a percentage of the population who appreciates the physical copy of the game. It will just depend on what goes down faster: supply of discs or demand for them.