Personally I don't see how some of the used markets listed as examples are comparable. In the game industry a used game can show up on the shelves within days of a new release, and then competes directly against the new release copy where there is no discernible difference between the quality of the two, for only a few dollars less. This is the business model Gamestop has shifted to over the last 5-10 years.
With vehicles, the new car dealership isn't really competing with the old car dealership. Automobile manufactures really have a sweet setup going on for them in society, as a car is typically seen as a 'status symbol' in our society. If people can afford it they'll buy a new car, used cars are for those that can't, and I really doubt a persons thought process is "I'm paying $20,000 now, but in 5 years I'll be able to recoup $2,000 when I sell my car." With gamers its a matter of weeks/months when you recoup your loss, car ownership it's years. Also when someone buys a used car it's going to come with a greater risk of breaking down and requiring some work, which means the mechanic will probably be ordering parts from the manufacturer so they're still making money off of it. So do they really take resale value into account when selling you a car? With so much real competition it seems like something that would get weeded out.
Also music, I don't think anyone's going to care about being able to resell a single song. It works on iTunes because you're starting with something that's such low value.
Anyways, it's pretty obvious why video game prices have come down. If I make a game that costs $1,000,000 and will sell 100,000 copies, then make a game that costs $100,000,000 but will sell 10,000,000 copies, sell them at $50 each and you've made the same amount of money. Video games are software, and with software the vast majority of the costs goes into R&D and marketing, the actual physical copy you produce probably costs pennies. It's all about how much you're going to sell, and the video game industry has grown a lot over the years. Also factoring into the costs they probably saved a ton switching from cartridges to discs, those old cartridges probably did cost a few bucks to make back in the day.
So the real question for today is, has video game sales/growth stagnated? Development costs have gone up, but if people aren't buying more you have to either raise prices, cut your budget, or take a loss. And no one wants to be the first to raise prices. But then again, if it all goes digital I doubt anyone will want to be the first to lower prices as well. Much easier to say "game development cost has skyrocketed, we need the extra savings from cutting out the middle man to stay in business and continue making AAA games". Now I'm totally onboard with Steam, but don't forget that Steam is a privately held company. Gabe Newell can do whatever the **** he wants and doesn't have to answer to anybody, so if he believes in passing the cost savings onto the consumer then we when. All the other big publishers and console makers we're looking at are usually publicly held corporations, meaning they need to answer to their shareholders and shareholders always want the immediate profits. They're only going to do what Steam does if it makes them more money, and someones going to have one hell of a time trying to convince them that it does... and maybe they'd be right it doesn't.