Every high growth era is defined by the abundance of "new adopters" - people who do not have habits or who are willing to change their habits. What happens with populations is that they do things a certain way for long enough periods of time that they don't want to do it another way.

This has little to do with Chinese policy, and a lot to do with the fact that Chinese life is moving from extremely rural/poor to middle class and thus, had no prior opportunities to do things like reading physical books, so no habits or preexisting preferences to overcome.