(I read all the first seven pages of the thread. I may reuse and repeat some ideas if they match mine, with no references and direct credit provided. Still, I believe there are some new thoughts on the following.)
If the decision on upheld, and then expanded to the EU and the rest of the world, I think it would be a good thing (there are three subscenarios in this case). Not because it would necessarily change prices, supply and demand, but because it would force the industry (we focus on the video game industry, but it also affects music, movies and software). Let's say this this goes into effect.
1) Then buying a game online allows the user to sell their game. The process should be something like that:
a) There are new games which are sold from Steam, which people can buy, to become an owner of a license of playing that game [I'll say "own the game" for simplicity].
b) A legit owner of a copy can sell their game, typically offering in a online store. Steam would likely offer this service (of facilitating sale for a fee) through a service of its own, like eBay, but it would not be able to be a monopoly (because if it tried, it would be sued for monopolizing the market and not really allowing sales of used games). Other services would allow it, with different fees and features. It would also be possible to sell it by person-to-person, at no fee, say it by transferring a encrypted file or something.
c) They can sell for any price they want. It would be less or equal that the price the original copies of the game (original meaning that the buyer would be the first buyer; won't clarify it again) are selling for at the official Valve/Epic/GOG store (in case the original distribution ceases selling, then the price could rise, the same way that happens with physical copies no longer printed, according to the process that is commonly called supply and demand). At the same time, it would not be 1 cent. It would be a fair compensation for the seller to lose the game and the buyer get the game.
d) Upon sale, the current owner of the game irrevocably loses the license to play the game. This would need to be implemented technically by Valve and other stores. (I can see people try to circumvent whatever the method is, trying to make pirate copies, specially now that they legitimately hold the game for some time in their possession. But there are two objections to the destructiveness and novelty of the issue. First, piracy already exists, with physical and digital games. If piracy is effective in preventing people from buying legit copies, the price adjust itself, like it happens with physical games, or even GOG, which allows the files to be downloaded DRM-free. Second, that happens also in other markets, like books. What if I buy a physical book, scan all the pages, then sell my book? Same thing. Note that piracy allows one to have a lesser copy of the product. Even for a digital game, the moment it's pirated, can't be sold as a legit copy [or rather could, but it would be fraudulent].)
e) The new owner would get a legit copy of the game. Used, but legit and legal and complete. I saw people arguing there could be a special gift, let it be a skin or weapon or another piece of in-game asset for the first owner only. I don't believe it would be possible to make it illegal to sell whatever it is to the buyer along with the game. As such, there could be timed offers of exclusive in-game assets, for example with pre-orders, but it would not be possible to make first-owner-only offers.
2) This would certainly affect the prices of digital games. There are two reasons in which digital games differ from physical games. The first is the ease of creating and distributing copies (currently easier than printing media and sending to physical stores). The second is the fact that the digital games can't be resold (at least not without many conditions). If games can be resold, it will be something like the market for physical copies. There will be fewer of very drastic discounts, which entice you to buy something you'll possibly never play (I guess many of us have enormous libraries with hours and hours of untouched games), and more of a steady decline of the price of the product. At the same time, an original copy may be sold for more than $60 (which is a common price for brand new, just released games) as being able to sell the game is a feature the first buyer would be purchasing with the game. With a new regulation and with price changes, it alters the industry a bit. That is, what was profitable before, may not be. Other options may appear (streaming, and others no one has ever thought about). The market would adjust. Probably to the better to some but not all customers, depending on their spending patterns and many other factors.
2.1) It is important to note that I don't believe that the games that are currently owned will be able to be sold, and that was the hypothesis I was working with. I think that it's more likely that the court decision, if upheld and expanded to other countries, would apply only to new purchases of games, not retroactively. Either way, it would affect prices, not invalidating what I wrote previously, except that it would in general push the prices down a bit (after all, supply is being created).
3) Now I believe that there could be a new modality of sale: loaning/rent/borrowing. That it would work exactly as it does today, but now it would have it's correct name: you are borrowing a game for unlimited time, but are never the owner of the game, and can't resell it. Let's examine the three possibilities (the fourth is that digital selling of games disappears, extremely unlikely), called A), B) and C), depending on the legality of selling and lending.
A) It would be possible for a customer to either buy or borrow (for unlimited time) the games. That is, when interested in an original game from Steam, the user can either buy the game, which would allow him to sell it, or to borrow it for unlimited time, in which he would be constrained by the restrictions of today's digital games. Obviously borrowing would be cheaper. This would be the best case scenario, and actually the most likely if the court decision if upheld and expanded. The rule would be: it is legal for Steam and others to lend a game as long as they can purchase the complete, resalable game.
B) The court forbids borrowing: only complete sales are allowed. This case was treated in 2). I believe that since, for example, one can at the same website or store have offers of selling and lending a physical book (at different prices obviously), it would be true also for digital games.
C) The court forbids the practice of lending a game and calling it a sale, but it allows the lending of a game without the need of a sale offer. In that case, the market would be pretty much as today, but we would refer to the acquisition of nontransferable licenses to play games as borrowing a game (or maybe getting a game loaned, or a new word that hasn't been invented) instead of buying a game.
4) I believe that in any the three scenarios A), B) and C), we would be better off. A) for the variety of options, B) for the consumer protections and C) for the honesty about the transactions.