Your first assumption is that China (which has variously risen and fallen and risen and fallen) will not rise to the position it normally has been in over millennia, which is the most powerful economic and political entity in the world. (The Roman Empire would have made a nice province in China)
Your second assumption is that all the various minorities (Mexicans, Muslims, etc) will assimilate and become Americans and not keep their 3rd world cultural values, even as they eventually eclipse the white population (the source of traditional American values)
Your third assumption is that the current white culture possesses enough discipline and vigor to match the discipline and vigor of previous generations (such as WWII generation) such as to keep America dominant, let alone even remain a viable political entity in the future.
Oh boy. Our “white” population is an amalgamation of many different European cultures which were as diverse and opposite to one another in their values as you can imagine and knew it. If you think the Irish and Italians were welcome in this country - you should brush up on your history. Most of them practiced their own respective culture, missed their own country, while at the same time strove for citizenship because that’s what they were expected to do. None of these loosened immigration policies.
I believe most of those immigrants agreed about one thing - liberty. Well, until recently, of course.
My point is - that’s all that matters. If you’re a purple people eater you’re welcome in the U.S. as long as you get your dang citizenship and believe and practice that one simple concept - liberty.
There is a problem with assimilation right now, but that’s because nobody is being forced to get their citizenship and stay here. It’s all being played fast and loose.