> Are we sure per capita GDP is the measure we want to use, though?
Oh I completely agree that GDP is a terrible measure of prosperity (and even median income is lacking, as you noticed). But I was not using it to measure individual/median prosperity, but the overall material wealth of the country, to debunk the assertion that a single teacher's salary could comfortably sustain a family then only because of a unique, brief set of circumstances that materially enriched the US.
We always have to be careful with per capita averages. If one billionaire increased his prosperity by $10B, and the other 400 million people decreased theirs by $10 each, we would say the country's average prosperity increased.