>The American melting pot works well (or worked well) because it was a nation made up from a blank canvas with no prior historically established dominant ethnicity

That's a bit of an oversimplification. They were British colonies for well over 100 years before declaring independence. The US Census website states:

"Not surprisingly, the first census reported that based on the names of heads of families, more than 90% of the White population in 1790 hailed from British stock: English (83.5%), Scottish (6.7%) and Irish (1.6%)."

https://www.census.gov/library/stories/2023/12/boston-tea-pa...

>And even at that was built on first disenfranchizing (to put it midly) the natives.

Not many European colonial powers purchased land from natives the way the US did. For example, considering the Louisiana Purchase area, the US paid over 20x as much to natives living in that area as the US paid to France:

https://www.slate.com/articles/news_and_politics/history/201...

The US looks bad compared with a hypothetical (nonexistent) perfect country. But compared with European powers, it looks pretty good.

> Human change can be subject to the law of large numbers, but nothing necessitates any particular change being towards progress.

The same can be said of the "Great Man Theory" (or its adaptation by selecting immigrants based on some selected set of skills). You don't know that you're making society better, you're just selecting for a certain set of skills.

> The American melting pot works well (or worked well) because it was a nation made up from a blank canvas with no prior historically established dominant ethnicity or culture the kind other nations have had going for millenia.

This isn't true and it ignores the cultural differences amongst all of the original colonists (religious, language, political, and country of origin). That's before you even consider the stark differences in culture between the Native Americans and the European colonist.