Trouble is a lot of gamers get the money to pay for new games by trading in old ones. You take away the ability to trade in a game and those gamers will be less frequent customers. You also take away the opportunity for gamers to play rare old games that they've heard are fantastic but can no longer pick up new.
And it's dispute the fact they don't make money off gamers who buy or sell games second hand. There are a lot of ways in which they make money off them now, whether that's charging for the online pass or the DLC almost every game has now. Not to mention, as I pointed out, plenty of sales generated by the fact gamers can trade in the games they've finished in order to afford the new releases. And in the end what's better for publishers, being able to see a new release for £40 on day one or having customers wait 6 months and picking it up in a sale for half that?