Yep, that's the one place where digital downloads are a boon for content companies--it stands to kill used sales.
As you note, you can't buy a used copy of software as you're buying a license.
Publishers, video game companies etc. are drooling at the thoughts of going to digital download and driving the used book stores, Gamestops etc. out of business and not losing so many sales to second hand sales.
But the risk is piracy makes it much easier for people to get copies for free than it is with physical media where you either buy new, buy used, get a copy from a friend, or steal a copy. Now you can just go online and download a copy of something you didn't pay for, which feels like less of a crime/moral wrong to most people.