I'm guessing you don't mean "natives" in the modern sense. But I am still one of those people who's ancestors have been in this country since the 17th century,

I'm guessing you don't mean "natives" in the modern sense. But I am still one of those people who's ancestors have been in this country since the 17th century, and I don't get what worries you. I was always taught that being American is not about being a particular race or ethnicity. Heck, my family couldn't even tell you our ethnicity other than "white". And I may be educated middle class, but my grandparents who grew up in the Appalachian mountains sure weren't. So long as the children of immigrants are adapting fine to the rest of American culture, who cares if our population growth depends on them?