no the US will not end up like South Africa because in the US, the intermarriage rate is high.
from a pew survey on wikipedia:
The study found that in 2008:[12]
A record 14.6% of all new marriages in the United States in 2008 were between spouses of a different race or ethnicity from one another.
This compares to 8.0% of all current marriages regardless of when they occurred.
This includes marriages between a Hispanic and non-Hispanic (Hispanics are an ethnic group, not a race) as well as marriages between spouses of different races be they white, black, Asian, American Indian or those who identify as being of multiple races or some other race.
Among all newlyweds in 2008, 9% of whites, 16% of blacks, 26% of Hispanics and 31% of Asians married someone whose race or ethnicity was different from their own.