Since anything really original or intelligent will always be above the common man, at some point the advertizing revenue will plateau below the level made off popular but totally stupid shows.
This lets the networks know that they are reaching too high in the gene pool, and so they re-trench.
PS: I like Futurama a lot more than the Simpsons, but the viewer base can never be as large, because the concepts, while bolder, are less universal.