How many times does an argument have to be wrong, and for how long, before it finally loses credibility? I suppose the answer must be nearly infinite, because the "they will not assimilate" argument is rising again, despite being about 0 for 19 on the groups to which it has been applied. Germans, Irish, Italians, Eastern Europeans, Chinese, Mexicans and now Chechnyans. This argument always seems to be treated seriously in real time and then looks stupid 20 or 30 years later. As an extreme example, here is Benjamin Franklin writing about Germans in 1751:
why should the Palatine Boors [ie Germans] be suffered to swarm into our Settlements, and by herding together establish their Language and Manners to the Exclusion of ours? Why should Pennsylvania, founded by the English, become a Colony of Aliens, who will shortly be so numerous as to Germanize us instead of our Anglifying them, and will never adopt our Language or Customs, any more than they can acquire our Complexion.
(By the way, if you want to retain an unadulterated rosy image of Franklin, who was a great man for many reasons, do not read the last paragraph at that link. People are complicated and sometimes even great men could not shed all the prejudices of their day.)
The only good news is that the circle of those acceptable to the xenophobic keeps getting larger. It used to be just the English, then it was Northern Europeans, then much later it was all Europe and today I would say it is Europe and parts of Asia. So that's progress, I suppose.
Fun fact: Ironically, the English King at the time Franklin wrote the quote above was George II. He was actually a German immigrant, born in Germany before his father came to England as King George I, jumping over numerous better claimants who were Catholic. His son actually assimilated very well, as George III spoke English as a first language, and his granddaughter Victoria practically defined English-ness. By the way, Victoria would marry another German immigrant.