In effect, they're being brought here to replace the 50+ million Americans we've killed off since Roe v. Wade.
Although immigration drives 80% of our population growth, this country has no shortage of population. In 1970 we had 203 million people ; today it is 317 million. In 1970 one in 21 was foreign born; today it is one in 8, the highest in 90 years; and within the next decade it will be one in 7, the highest in our history.
There's no reason to lament the destruction of a nation that has done so much to cause its own demise.
When historians write about the rise and fall of the American empire, they will point to the Immigration Act of 1965 that sowed the seeds of our destruction. Demography is destiny.
The U.S. already has the highest standard of living in the history of mankind. If we import large numbers of people on the left side of the bell curve (in terms of income, net worth, or any other financial measure), our per-capita standard of living is likely to decline. But that decline will not be across the board. It will be the result of importing an entirely new lower class.
In effect, what has happened in the last 50 years is that the American middle class has largely disappeared -- by rising into the upper class. This does not mean that everyone is extremely wealthy, but when you look at how the "middle class" lives today you'll find that even the wealthiest people in the world couldn't live like that as recently as a few decades ago.