Excellent points that refer to the decline since the 70s, but I would argue that America was never a particularly prosperous country.

It got a small taste of what prosperity might look like between the 1950s and 1970s when it toyed a little with the idea of government intervention in the economy and social welfare, but it rejected that in the 1980s and hasn't looked back.

Prior to WW2, it was a pretty backward place, with the exception of a wealthy band of robber barons and an upper class that mined the rest of the country for all it could get, relying on ultra cheap or slave labor to win the day.