The one risk I see is what happens when a game's been on the market for a year or two, and you can't *find* new copies anywhere. Then you're forced to go the Gamestop route. If Microsoft theoretically does this, that means that publishers will have to keep up the supply of games or people won't be able to find them at all. And while I know they don't directly make money off the second-hand sales, they might still end up losing out on potential revenue from DLC purchases as well as just making consumers frustrated at not being able to find what they're looking for.
I've sometimes wondered if that hasn't driven used sales a bit. When I'm buying games for my daughter, I always try to buy new games because they're usually gifts. But occasionally I've run into titles that have been out for several years that I wanted to buy her that I couldn't find new anywhere local. I could sometimes order online, but I couldn't find a new copy in stores to save my life. Perhaps they'd do a better job of this under the locked-to-console model.