Exactly as it says on the tin. Is ethnic Homogeneity really necessary for a successful country? I have often seen people argue that this was the reason why Western Europe, Japan and later China were able to successfully industrialise. If so does that mean non-homogenous countries are fundamentally incapable of developing nationalism and having a strong economy?
Britain was one of, if not the most, successful industrialising nation. Do English, Scots, Welsh, and Irish count as a single ethnicity? (Legit asking).
Also, Germany included large minorities of Danes, Poles, and French.
And then there's ethnically homogenous, but largely backwater for most of the 19th and 20th centuries, Portugal.
Part of the problem is you're taking the last 200 years, a brief snapshot in human history, as somehow definitive. The Great Divergence was a fluke which is even now fading away. It should not be used as definitive of anything.