[Majromax]

> If true, then when slavery was ended we would presumably have seen a stagnation in the economic fortunes of the USA, which I don't think has happened.

Something like that did in fact happen -- the post-civil war period coincided with the Long Depression (https://en.wikipedia.org/wiki/Long_Depression), which some economic historians would date between 1873 and 1896.

However, the Long Depression was at least in part a global phenomenon, and with any economic story there are many confounding factors. (And on top of all of this, nations did not keep rigorous economic statistics at the time anyway.)