To be fair a lot of it had also to do with the sheer immense amount of vast, mostly unused ,fertile land available in north America. I sincerely doubt the American experiment would have worked this well if they had rowdy neighbours and infighting due to resource constraints. For almost 200 years the solution to most things in the USA was to get a chunk of either their people or immigrant to move to the neck of the woods to find fortune
Not getting wrecked in major land wars during the 1800s and 1900s also helped
> sheer immense amount of vast, mostly unused ,fertile land available in north America
it was not "available", it only became "available" after we killed off nearly all the inhabitants and stole their land
But the success hasn't ended since the unused land became taken; in fact, the US became a superpower after the westward expansion era. My point is that looking at conditions today, the US still continues to succeed (by some definition of success) and other countries should try to emulate the aspects of the country that leads to that success. IMO one of the big factors is how well immigrants assimilate in the country, and birthright citizenship is a part of that.
I do agree with you that US success in the 19th century was due to many factors that are not relevant today.