The founders were, themselves, immigrants. From multiple different countries.
Of course racism existed when the founding father's were alive, we had slavery. But, from the beginning, the US has been a country founded on immigration and racial diversity. Yes, we did not, at the time, consider black Americans Americans. But we certainly do now, and have for a very long time.
I'm also not convinced immigration lowers wages. People throw this around, while providing no evidence. And, even if it did, I think it's obvious that wages is not the cause behind birth rate decline.
All the developed countries, with the high wages and standard of living, have poor birthrate. All the developing countries, with low wages and low standard of living, have high birthrates. Hmm.
If we look at the demographic progression of nations the "culprit" behind birthrate is development. Particularly, social development, the liberation of women. Turns out lots of women don't actually want to have children, and when we stop pressuring them socially, they don't. They work instead.
Obviously this is a hard problem to solve. If you want to solve birthrate, you need to de-develop your nation. Not good. Or, some other solution, which nobody has found yet. Or, you just have immigration, which we do and have had.