Yes, and no.
I'm going to use Milwaukee and an example, because A) its one of the more successful examples of a sustained German-American culture and B) because its the one I'm most familiar with.
Germans were present in the city, in large numbers, from its very founding - many of them liberal refugees from the Revolutions of 1848. From this point onward, if they did not outnumber the English-speaking population of the city, their numbers were very nearly equal. As a result, the need to acculturate was never very strong - they simply began to set up German-language institutions which paralleled (and often times surpassed) the English speaking institutions. Milwaukee became a center of German language theater, literature, political thought and so forth - not just in the United States, but recognized throughout Europe as well - it came to be known as the German Athens. This was actually strengthened by other immigrant groups, many of whom, like the Czech and Poles, probably spoke German as a second language long before they adopted English.
The result of all of this because that, from the 1850s through the 1920s, there was simply no reason to fully culturally assimilate. The Milwaukee Germans DID learn English - by the turn of the 20th century, all but the newest immigrants would have spoken English as a second language. However, the prevalence and strength of the German institutions meant that one might use English in one's business or political life, but German would remain the language of choice in every other aspect of life.
World War One, the anti-immigrant hysteria of the 1920s and, finally WW2, are what really brought an end to this way of life. Prior to the war, there was no evidence that the culture was weakening in the least (the fact that the Germans were at the forefront of the city's Socialist party and its coming to power actually argue the exact opposite.)
I think you are still attached to the old "melting pot" theory of immigration, which has been largely disproved (or, at least, heavily, questioned) since the 1960s and 1970s. If you are interested, you should pick up Gerde's "The Minds of the West" which covers immigration to the Upper Midwest and West in the 19th and 20th centuries. Its an amazing book and one of my favorites!