The median worldwide income is up a lot more, but I do not have an easy graph for that.
The inflation adjustment is supposed to proxy for all the various utils your dollars (or shekels or yuan or rupees) can buy.
In practice, most people will accept this at least for a base point of argument as alternative measures tend to be even more controversial. Even those who have profound issues with money, price, value, inflation, wealth, and more. Such as myself.
A big part of the reason for that, was every US competitor got destroyed in WW2. The US inherited an open runway, and acquired over 50% of all global manufacturing at the peak accordingly.
As major countries, like Britain / France / Germany / Japan got their feet back under them, that competition drained the easy income and wealth off the top of the labor force in America. The rise of China, Brazil, India, etc and the opening up of the Soviet bloc has further stressed the US labor market, by increasing competition.
The peak that you're referring to that America has been coming down off of, was temporary and would not normally have existed had it not been for WW2. It was a fluke.
That massive concentration of income and wealth the US acquired due to WW2, has been gradually redistributed due to competition. People in Estonia, Norway, Finland, Denmark, Sweden, Norway, Czech, Slovakia, Brazil, China, Australia, Canada, etc. etc. have seen massive increases in their standards of living the last 40 years.
The highest (adjusted for inflation) minimum wage in the US was in 1968 and comes out to $10.71/hr in 2013 dollars [1]. That's about $21,500 per year.
Where did you get the $35-40K number?
[1]http://money.cnn.com/interactive/economy/minimum-wage-since-...