A million people a year isn't much for America which has 300 million people. Canada for example is letting in 250k for a population of 30 million.
I've seen graphs of how it brings down their average age, and brings up the the worker-dependent ratio even much more.
Into the future, like 30 years I dont' think most politicians think that far ahead. I would assume by that point they will be looking at automating away a large portion of the work. ---
Or even better if they can stop abortion, and rebuild the family, plus limit hte benefits going forward.. there wont' be an issue to worry about.
A million a year plus massive illegal immigration is an enormous amount for any country to take in. That reasoning, which we hear frequently that it's small in comparison to the population can also be turned around to highlight how those same percentages could mean 10 million annually in China, or 1% of a billion. Do you think China needs another 10 million people a year added to its population ?
In 1924 we gave mass immigration a rest and it's long past time to do it again. That legislation proved to be very successful at assimilating those immigrants into the mainstream of American life.