Tell that to the native Indians, the black population, the millions of Hispanics living in the shadow. Even FDR's New Deal excluded nearly all of them. What thrived was a
white middle class. America is a country built on exploitation and aggression, and when that tendency turns inwards when offered competition, it destroys itself. The Pax Romana was a thing for 200 years because Rome could keep on expanding and levying its conquests into new sources of income. The moment they turned defensive, events like Year of the Five Emperors became much more damaging. We are just seeing history repeating.