I've always felt that this was a very intellectually dishonest way to look at things. Has there ever really been a period in history when the majority of the population was highly bookish? Books have been around for centuries and literacy has only been since the advent of the printing press. The popularity of e-readers the rise of the superstar author is proof of this.
Sure, we may not spend all our time reading David Foster Wallace and Pynchon, but is this any different from history? Was the typical farmer or craftsman in 18th century England reading Kant? People generally read what is familiar and easy to them. Which rather underscores what Lewis is saying about reading children's lit.