America already has large ethnic populations and it hasn't prevented it from being the success story it is. There have been similar scare stories regarding race, the growth of Germans, and Irish, Italians then the argument moved toward color; Black, Asians, now Hispanic.
America has consistently integrated new communities, Obama's election was a sign of the great change that is taking place, not to mention that communities are increasingly intermarrying.
"Race" is inherently a false way of categorizing the human beings from a scientific point of view, since we all belong to the same species, and a relatively young species at that.
Culture is the only valuable way of differentiating people, America is historically by it's very nature as a young state a success story of absorbing diverse cultures which produce a distinctive "American" flavor. This next chapter of the American story ought to be celebrated.