Again, I'm sorry but that is simply incorrect. It is an opinion driven by your preconceptions of the past and the present and is not an accurate representation of reality, then or now. You are expecting today's immigrants to assimilate instantly, which is functionally impossible to do on a universal scale and is not what happened, again for the most part, historically. You can cling to your opinions all you want, but that is not what happened in the past nor what is happening in the present according to the totality of the data.
Think of assimilation as an oak tree growing from an acorn. You cannot track it day by day, month by month or even year by year, but in 50 years, you're going to have a big tree and an assimilated culture. You must also keep in mind that assimilation does not mean wholesale adoption of our culture. The immigrants of history who assimilated also injected their cultures and histories into America's evolving culture. If not, we'd all still be Puritans and we certainly wouldn't have the diverse culture that we have now. So assimilation of the "new" immigrant cultures (and they're not actually new, as this country has been exchanging immigrants with Mexico and other Latin American countries for generations) will mean an evolution of American culture, and therefore I'd suggest you get used to that reality, even if you don't choose to like it.