Again, LOL.
Not a big issue? You think that JFK just wrote that book in 1958 for nothing, and that only a quick seven years later the most destructive legislation in American history was signed into law?
There were two reasons for the McCarran-Act, and both similar to the restriction acts of thirty years before. One was a fear of communism, the other the desire to maintain the ethnic balance of 1920. But now there was a recognition of the needs of political refugees, of the relatives of war brides etc. There was also something you forget: the successful integration of the children of the earlier immigrants, so that the earlier fear of being swamped was lessened. One of the cliches of war movies and movies about the war made afterwards, was the platoon consisting of a Kelly, a Picard, a Krauss, and Polaski, a Martinez. The American power to assimilate seemed strong. The Baby=boom swelled the ranks of the americanized. Who know that the next generation would stop having children?