There is no way that that restriction is going away after Gamestop is dead. There is no way MS will give up something that increases their profits. I'm thinking the mechanism will be some rewritable disc section that the system will destroy after reading its code and linking it to your account, that would not require an internet connection to determine whether a disc is new or used.
BTW, I'm pretty sure used games produce the majority of GS's profit but probably not their revenue. Revenue is the money that goes into the cash register, profit is the money that stays in there. Since new games are pricier they produce more revenue but because their purchase price for GS is also WAAAAAAY higher (I heard that new games come close to loss leaders for stores, merely getting people into the store and hopefully buying something else) they produce very little profit.
Old game preservation will be fucked by no-used-sales. At least on the PC there's piracy that preserves games in a form that works independent of the publisher but on consoles there won't be a way to get old games, at least not until emulators can run the roms. Yes, they may be on sale online but those offers are not permanent with issues like licenses expiring which has happened to several games this gen alone. If this technology had been available back in the 80s then games like Ducktales would be lost to mankind forever.