The expansion of the middle class has nothing to do with the top tax bracket. It has to do with industrialization before automation (lots of low/medium skill jobs that provided significant value), and the destruction of competition around the world.
Speaking more seriously, I'm pretty certain the expansion of the middle class was due to strong white labor unions and Fordism as a cultural subtype within Modernism.
This entailed many things, with which the top tax bracket had little to no relevance.
