This is just the classic old flawed argument:
This is missing a huge part of the equation. Where did GameStop *get* that disc to resell in the first place? It didn't magically appear. They didn't counterfeit it. It's a new game that was sold to someone at full price. And that consumer -- especially given how quickly they traded the game in -- is not stupid. They're factoring in the game's resale value when they purchase a new game in the first place. It's not necessarily a "lost sale" in the aggregate, which is what we are talking about, because some percentage of that cash was only spent in the first place because it was being spent on a durable good with significant resale value.
And then, of course, there's a good chance that guy got the money to buy your new game in the first place because he traded in his old games.
The point is that this is a complex situation, and that this idea that there is a 1:1 relationship, that a used game bought is a "lost sale," is fallacious. I'm not saying for sure that it's a net negative or a net positive once all the dust has settled, I'm saying it is not knowable and that Microsoft and publishers are messing with something that nobody truly understands because we've never had an A-B test.
Again: Not necessarily. It is entirely plausible that the existence of a robust used market could be an overall boost to the fortunes of the game platform, since cheaper games mean a lower barrier to entry, which means more customers, which means more revenue for Microsoft from consoles, accessories, Xbox Live subscriptions, etc.
So again: Extremely complex system, impossible to unravel with any certainty. I don't think Microsoft is concerned that they're losing money off used games sales, I think they have a broader vision that involves bringing all game ownership permissions into "the cloud," which necessarily means moving the notion of "possession" from the disc to the account.
Used games are not the target, they're an innocent bystander.