Is that really true?
Because I can think of dozens of counterexamples. Like the Basilica of St. Josaphat in Milwaukee - where the (gorgeous) dome has an inscription in Polish
What about the Amish of Pennsylvania's Lancaster County? Who are still speaking a version of Alsatian German a hundred fifty years after the last immigrants arrived?
Milwaukee had more German language newspapers than English-language ones up until the mid-1930s.
Or the Cajuns of Louisiana? Whose ancestors arrived on these shores as Acadians in the
seventeenth century, a good hundred years before the founding of the United States - and yet still have significant number of people who speak their own unique dialect.
Get back to me in twenty or thirty years, and show me large communities of Hmong, or Vietnamese, or Koreans, etc. whose children and grandchildren still are speaking in those languages.