“White replacement theory” is a quantitative discussion.
The population numbers show that the black population in the country has not significantly increased in recent years.
So it is not a white/black issue.
I leave the rest of the discussion as an exercise.
It’s only a “quantitative discussion” if you harbor a belief that it is significant in some way; that there is no content to American values apart from whiteness. If you believe there is something compelling about American civic life that might be attractive to people from elsewhere and make them want to immigrate here, then why does it matter what skin color the people have? Explain it to me like I’m five.