There are plenty of very successful single player games these days... but exactly like you said, they tend to involve the player interacting in unique ways to make the experience more varied. Grand Theft Auto and Red Dead Redemption, Elder Scrolls games, Far Cry and Just Cause. Even Mario and Legend of Zelda have gone this route. Linear story-driven single player games are niche, but open world, player-driven, interactive, "radiant AI" style games bring the feeling of a living multiplayer world to the single player experience.

Not disagreeing with you, just adding on.