Yeah. I think that a lot of these changes after 1940 are much more to do with socio/political factors than growth. By 1940 you have the GDP to support a middle class. It's the collective experience of the Great Depression, WWII and the rheotric of the New Deal that produce a populace that just wouldn't put up with significant imbalance in the riches of society.

People who fought in and justified WWII contrasted it with WWI. The claim being that WWI was a war fought on behalf of the old world system. And WWII was a true war for Democracy. I think it would've been very hard after that kind of mobilization, sacrifice and rhetoric to re-instute the Gilded Age ethos of a system which the rules of the game were sacred about the outcomes.

In this way American Democracy is born of the FDR era. I don't mean this in the narrow sense of the size of the electorate though that is important. At every level, America's systems Democratized. From the way the Supreme Court interpreted the law, to the choices of industry in what to produce, to how the rewards of production were distributed. There was a broad transition from the notion that institutions existed to maintain rules towards a notion that these institutions in their largest sense needed to serve the people.

I think a lot of what has happened since 1970 is a transition back to a way of looking at the world through the pre-democratic lense. The biggest difference being that now there is a plurality which is the upper half of the middle class who have some similar interests to the wealthy in protecting their wealth.