It's been said that the single-player game is dead, though whoever said that was a bit ahead of the game. Even so, the past few years have seen a huge increase in multiplayer gaming and almost every game now released offers multiple player options, network support or a connection to an Internet hosted game. The problems are that no one has really worked out how to make money out of it yet, and the hardware is just not fast enough for the games we are producing. If you go to watch a movie, and you're sat there in the dark, transported into this amazing world, then you really don't give a rat's ass about the person sitting next to you. It's great to play against, and in co-operation with, others, but it's equally great to be part of an unfolding story, where every twist and turn has been expertly written and directed, such as Final Fantasy VII or Zelda.
People watch TV by themselves.
They read books by themselves. I don't think people are suddenly going to want to give up their 'solitary' entertainment experiences.