[jordan0day]

"... the value of the disk to that person is now zero. Given the opportunity, they will sell the game at any price they can get. They will happily trade the game in for $5."

But that's not really true, at all. "Replayability" is a term that gets tossed around, a lot when describing single player games. If "the value of the disk to that person is now zero" were true, it's a term that wouldn't even exist.

Additionally, I hate to resort to anecdotes, but I'm not sure if there's any unbiased research in this area -- but I don't know anyone who buys a brand new game, plays it for a month, and then "will happily trade the game in for $5." I mean, I'm sure there are people who do fit that mold, but if that were true, then all the used games shops would only give you $5 for any month-old game. But those month-old games command a much higher price than that, precisely because people aren't happily trading in the game for $5. This is really basic econ 101 stuff here. Supply and demand.