In my opinion, people who think that bilingualism is going to permanently divide America ignore both history and social science.
Looking at the evidence from history, there is no reason to think that bilingualism will make for a permanently divided America. The present day is not the first time in the history of the United States that we have had many immigrants. It is not the first time that the first generation immigrants have had a hard time learning English. It is not the first time that many of their children were bilingual. None of the previous waves of immigrants and the bilingualism they brought with them divided us permanently.
Looking at the evidence of the social sciences, it is hard to believe that bilingualism will split us. The second and third generations of immigrants today are just as likely as previous waves of immigrants to speak English. These descendants of immigrants are assimilating just as previous waves did. They are intermarrying and generally behaving just as the idea of the “melting pot” says they will.
Bilingualism and unassimilated immigrants are temporary phenomena. They are not likely to permanently divide the country.