I enjoyed your discussion of the possibility of having a "forever" game. I can't imagine having a game in which I'm playing against the computer fill that role, but then I'm a far more avid tabletop gamer than I am a PC gamer.
In that vein, two games I know I'll be playing for the rest of my life are Star Fleet Battles and Advanced Squad Leader. Admittedly, both tactical games, but the depth of depicted detail means I'll probably never know -- much less be able to master -- all of it, and the huge breadth of scenarios for each of them means I could face new challenges and narratives all the time. Combine those features with the infinite unpredictability of human opponents, and it becomes much easier to see how some people gravitate to playing only one game.