Are you saying that 19th century America was poor? We were the richest nation on the planet. Yet our birthrate was phenomenal.
Moreover, name one "affluent" nation in the late 20th century that had real reason to be optimistic.
Affluence in the 20th century west has come with a real fear of growth. The "isms" of the 20th century put fear of life into us. Consider fascism, communism, and the new French philosophies -- each with their own "isms."
The west became afraid of its own shadow. The correlation of wealth to birthrate is only partly related.
So the American elites, and the western globalists have decided to replace our flagging growthrates with immigration, regardless of the cultural impact.
19th century America was largely agrarian, and we were NOT, I repeat NOT the richest nation on the planet. We didn't emerge as an economic superpower until well into the 20th century.
No, Great Britain was the economic super power of the 19th century and the English pound was the preferred international medium of exchange. The majority of Americans were dirt poor though we did have a class of very wealthy people. The rise of the middle class began during the war and continued during the 50s and 60s.