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.