Certainly --- just ask the American Indians --- assimilation doesn't always happen -- sometimes you get pushed onto reservations by the new group who won't assimilate --- they can tell you the dangers of open borders and unlimited immigration and waiting too long to worry about it better than anyone.
Well, precisely. And I imagine that many of the pollyannas on this forum don't know about Aztlan (charming how many hispanic people who are becoming our neighbors have a slogan of "For those inside the race, everything. For those outside the race, nothing"), or don't take seriously the threat of the Islamic conversion of America's blacks, in concert with the immigration of radical Islamics into the U.S.
No, I guess there's nothing different about this influx. So what that we'll be a nation of 500 million people towards the end of the century, and no culture will dominate. Everyone will get along. What will save us? The Constitution. After all, it's protection against any concevable ills, isn't it?