Something I want to touch on first, though, is that my example assumed direct seller to buyer transactions, while most used game sales in the real world involve a middle-man who siphons off some of the money involved. In such cases it could be argued that the money siphoned off by the middle-men is money that would end up going the devs/publishers if there were no used sales. However, it's important to recognize that this is not due so much to used sales occurring, but rather due to market inefficiencies that allow the middle-men to command higher margins than they would be able to in a more efficient market. The proper solution to seek in such a case is actually to make used sales easier- make it easier for buyers and sellers to connect directly, and make it easier for middle-men to enter the market, thus increasing competition between them and decreasing the margins they can command.
As for why eliminating used sales can actually hurt the industry, one has to look at the effects eliminating used sales would have on consumer behavior. One of the clearest and easiest changes to note is that while the amount of money being spent on games both with and without used sales is roughly the same, the amount of people able to play a given game decreases when there are no used sales. This means less of an opportunity for word-of-mouth advertising, and for multiplayer games it means a smaller player base, thus potentially reducing the percieved value of the game if it's known there aren't as many people to play with (this can be a big issue for multiplayer-centric games, particularly MMOs). Another very important thing to keep in mind is that while the total amount of money being spent will be about the same, the shape of sales over time will look very different. This is because used sales allow a fairly organic form of price discrimination to occur, and without them price discrimination instead must be carried out by whoever is selling the game over a much longer period of time. Basically not everyone will value a game at the $50-60 it's initially sold at. With used sales the price can either be reduced by used copies, or effectively reduced by the ability to sell a purchased copy, thus recouping some of the cost. Without used sales to do this people who don't value the game at the initial price will wait and only pick it up once it drops in price (assuming they don't just forget about it). Thus many of the sales that may have occurred within the first few months when used sales are allowed instead have to be picked up via long-tail sales as the price of the game gradually drops. This can have a couple of negative effects. First, many studios that don't have large cash reserves may be counting on those large initial sales to pay off various bills they accumulated during development, and having those sales stretched over a much longer time frame can put them in a tough financial situation. Additionally, many publishers will just look at the sales numbers for the first few months when deciding whether or not to bankroll expansions, sequels, or additional projects by a developer. While this issue can be overcome by changes in the way the industry does business, quite a few people will get burned while the industry wakes up to the fact these changes would need to occur. Another negative effect of sales spread out over a longer time is that, again, lower initial sales can hurt the overall sales of multiplayer games (due to the smaller player base), and can be the kiss of death for MMOs.
Ultimately, once you start looking closely at the economics of the whole thing opposing used sales is really a very foolish thing to do.