Exactly. Games (and movies, etc.) generally make most of their revenue within the initial release period, so it's not like only one person has to buy the game new then everyone else can just buy it second hand from each other, one after the other (ridiculous example to illustrate the point). So although a digital copy of a game won't physically degrade like a boxed game or a car or whatever, the game might still be six months old by the time you're purchasing it second hand so it has in effect lost part of its value still. Not only that but the reselling of games does potentially allow those buying new games to buy more of them and more often, which could offset some of the publishers' losses.