Without immigration, U.S. population growth would be well below replacement rate. This is a growing problem throughout the entire developed world with no fair answers -- increasingly, it looks as though the next several decades of geopolitical power will be defined by which countries can attract the immigrants necessary to maintain, at minimum, some population growth.
I won't pretend that the U.S. is a race utopia. We hit just about every single branch on the way down, if I'm being honest... but most of the ethnostates currently competing for world-power status haven't even realized there's a tree that they need to be descending yet. We have a powerful edge in the coming geopolitical era as one of the most pluralistic, multicultural nations in human history. We have unmatched institutions and experience when it comes to integrating immigrants -- not to mention the incredible advantage of having a language & culture that is familiar to the majority of living humans.
In my opinion, walking back from these advantages at such a critical turning point is probably not something that the country could survive in the long-term. We either embrace our identity as the world's melting pot or we wither away as hermits.