I have not read Bill Gates' book, but I have heard little dispute that the material conditions of the working class _in the developed world_ have not improved much in the last forty or so years. The "elephant curve" is the best simple depiction I've seen of this: https://www.economist.com/finance-and-economics/2016/09/17/s...

The article goes on to describe why it's not quite so bad as the original graph made it seem, but even the "Alternative" shows the same general pattern.