[From "The political answer"]

It's been 100 years since the Emergency Quota Act transformed the USA into the most powerful nation on Earth by limiting immigration and restricting it to compatible nations:

[…]
The Emergency Quota Act restricted the number of immigrants admitted from any country annually to 3% of the number of residents from that same country living in the United States as of the U.S. Census of 1910. This meant that people from northern European countries had a higher quota and were more likely to be admitted to the U.S. than people from eastern Europe, southern Europe, or other, non-European countries[…]

100 years later, it is clear that if the USA is going to survive another decade or two intact - which it almost certainly isn't - a new Emergency Quota Act is required, albeit one that restores the 1910 demographics. Since this is a political solution, and since four years of the Trump administration didn't even manage to seriously slow the invasive flow from around the world, we can safely discount any chance of this proven measure being adopted in any meaningful timeframe

The much more likely alternative is that those demographics are more or less restored in a rump state after a considerable amount of war and ethnic cleansing over the territory of the former empire. Unless, of course, China manages to establish a Chung Kuo-like hegemony over North America, which at this point is probably more likely that the sensible solution