I don't think you've really thought this through.
They are making a tonne of profit from only 10-20% of the total marketshare. A proper resale strategy however, would not only enable them to garner a much larger percentage of that market share and thus more profit, but much more revenue with far more purchases, enabled solely by those resale trades.
The resale market essentially promotes far more video game sales than would otherwise be the case, and that is why publishers cannot and will not forever ignore it, especially when they make zero profits from the retail resale market. There are many people like me for example, who would only be purchasing 3-6 games a year instead of the 10-15+ I currently purchase, as a direct correlation to the retail resale market. The fact that I can trade in new games, towards paying for other new games, is the only reason I'm buying so many in the first place. It's the only reason impulse game purchasing is even financially viable, and that will be the exact same situation with digital.
If publishers offer a healthy digital resale alternative, not only will they not be cutting in to their profits, they'll actually be making far more, and increasing revenue in a huge way. Essentially selling far more digital games than they otherwise would have, without such a resale strategy in place. As I said above, the reason Publishers have been slow to adopt any such system, is more to appease retailers than anything else, who still hold the vast majority of marketshare and who they do not want to be aggressively competing with.
This 10% offer from Microsoft to me comes off as them trying to take advantage of the current vacuum with the lowest possible low ball offer imaginable, in the hopes that it sticks, and thus secures far greater profits in the long run, at the expense of the consumer. We can hold out for better, but it's good that the conversation is at least taking place. Publishers stand to make a tremendous amount of extra revenue by offering a decent digital resale option, so it's only a matter of time.