But at the same time, the market for games has expanded greatly due to in-browser games, smartphone games, etc. Those people are "introductory/casual gamers". Some of them will remain casual, some of them will stop gaming entirely, but I think of those who continue gaming, many of them will come to want something more.
So, in short, the existence of lots of really shallow games largely reflects an expanded market IMO rather than some inherent shallowness of humanity. I share your cynicism in the sense that there are some who will never move beyond FarmVille, and that is sad, but I cannot imagine what mechanism would drive, or what evidence would support, the average individual gamer preferring less complex games over time.