In a recent interview with gamesindustry.biz, Epic Games’ president Michael Capps talked about this problem and predicts that the future of DLC (downloadable content) may be geared towards consumers who rent games or buy them used. He goes on to explain that Epic's primary retailer makes the majority of its money from second hand sales despite being a specialist games store, and while he refuses to blame gamers for this, it’s understandable that they want to compensate for missed revenues.
This is all well and good but later on he claims some developers are toying with the idea of charging a $20 fee to those who buy used games in order to let them unlock the full game – basically reducing it to a demo if they don’t. Capps isn't necessarily advocating this kind of move, but it shows how keen developers are growing on taking a cut of the lucrative used game and rental markets. The full interview with Michael Capps is available here.