Do you think a free person or an enslaved one produces more economic value? If we decided to enslave everyone with green eyes tomorrow do you think GDP would grow or shrink?
Just because a certain practices makes certain individuals wealthy does not mean it makes the society wealthier.
> self-selection were highly motivated and risk-taking
This is beliefs seems to be very entangled with belief in a meritocracy. If you believe hard work and risk taking doesn't pay off why work hard and take risks. If you believe hard work and risk taking does pay off, well than that sounds a lot like belief in meritocracy to me.
And if slave labor were an important component driving the relative wealth of the US, wouldn't you expect the abolition of slavery to decrease the relative wealth of the US?
But that isn't what we see in the decades after the abolition of slavery in the US: "In the last third of the 19th century the United States entered a phase of rapid economic growth which doubled per capita income over the period. By 1895, the United States leaped ahead of Britain for first place in manufacturing output." See also, "Gilded Age".
I thought it was the abundant resources, rule of law, respect for property rights, free slave labor (for the first ~200 years, including before independence), and wave upon wave of immigrants who through self-selection were highly motivated and risk-taking. The post-WW2 prosperity was largely due to a lack of competition from other developed nations.