What the publishers fail to realise is that the price they can sell new games for reflects the fact that buyers are factoring in the ability to sell the product on the used market. While some people buy to keep, others buy to play and resell. If you kill the used game market the price they can sell a new copy for will drop as all the people who were planning on reselling refuse to buy the game now. Only people who buy to keep will pay the currently inflated prices. (Unless they drop the price.)
The net effect is actually that the amount publishers make will be exactly the same regardless of whether there is a used game market or not. Or whether they take a cut from Gamestop or not (because any cut they take from Gamestop is going to be reflected in lower prices paid by Gamestop for used games, which is going to affect how many people actually buy the game new).
Lots of other industries seem to indulge in this short-sighted thinking. If you offer people an inferior product - use of something for a limited amount of time versus use for an indefinite amount of time the price they are willing to pay for it is reduced. This is even more so where people don't know what the value to them is and are taking a risk by buying it. If there is a possibility a game will suck, the ability to resell it on the used market makes it much more likely that people will take the risk. If the used market gets destroyed there will be fewer customers like that and, once again, the publishers will have to drop their prices if they want to keep making as much money as they currently are.