It simply won't happen. As home PCs become more powerful, programs become more complex, requiring more CPU horsepower. Take Supreme Commander, for example - a recently released RTS. The game is capable of supporting literally thousands of units on unbelievably large maps (the largest map that ships with the game is 6561km^2, and I think the game can -in theory- support nearly 100,000 units). Even though it's one of the few games programmed to take advantage of dual and quad core CPUs, it can bring even the most powerful system to its knees. On top of that, graphics continue to improve - texture resolutions will increase, 3D models become more complex. That's the main reason why your average game today required about 5 GB of hard drive space, versus the 700 MB of previous years.
Trust me, as long as there is CPU power to spare, programmers will find a way to use it.