He predicts this will be made possible by a unified architecture for computing and graphics that offers massive amounts of computing performance. While hardware will become 20x faster, game budgets will increase less than 2x, Sweeney concludes. Therefore, he predicts developers will have to sacrifice performance in order to gain productivity, and that easier hardware will beat faster hardware.
To make a forecast that far away in the future, Tim Sweeney makes bold assumptions about the evolution of computing power and about the quality of the compiler/tools that will be available by then. He also recognizes that development has to be economically viable and that there are many challenges ahead. Tim talks about "the end of the GPU roadmap" because at that point, there's no evolution in "features" anymore, but only in computing power. Roadmaps usually refer to upcoming features. This presentation was delivered in a context where Intel will introduce Larrabee, a graphics processor that gets rid of almost all (except texture filtering) fixed graphics functionalities.More info at Ubergizmo, the 74-page long presentation can be viewed over here (PDF).