I can only speak to my family's history and that I saw growing up, but my great-grandfather was an immigrant from a Slavic region (to be technical, Poland didn't exist yet, but he was a Catholic who spoke Polish).
To make a long story short, none of his kids ever spoke Polish. That wasn't familial pressure, but the physical and emotional abuse inflicted on children in schools to beat the slav out of them if they acted or sounded "fresh off the boat" - to the point where four generations later, all of us know that phrase because it's been said to us one way or another.
Contrast this with the most recent wave of Polish immigration to the same neighborhoods almost 80 years later. These are kids I grew up with. They had Polish clubs in school, they went to "Polish school," they spoke the language fluently and publicly, and to a certain degree that was celebrated by the rest of the community. The external pressure not to be Polish is essentially gone.
I don't have a grand point here, it's just something I think about a lot in these debates. I learned what the experience of my ancestors was, and saw something of what happened to my peers, and the only difference was 100 years. It seems to me the difference between what we think of as assimilation is really how likely a community is to join the melting pot depending on the internal or external pressures on that community.
Usa got lucky bcs it integrated mostly european of "similar" cultural values of its anglo base in a pre industrial society, and later integrated the brightest from around the world in a knowledge economy. Even Latinos illegals have familiriaty with US values.
Assimilation at the superficial level happens quickly. But deep culture—things like social trust or views on the relationship between people and government, are sticky: https://www.sup.org/books/title/?id=35594. The peak of continental European immigration to the U.S. was more than a century ago, but there is still a significant correlation on cultural attitudes between European countries and Americans with ancestry from those countries: https://cis.org/Richwine/Still-More-Evidence-Cultural-Persis...
Having the smartest people isn’t what makes a country great—which is why the super smart Indians and Chinese come to America instead of making their home countries rich.