He has endeavoured to prevent the population of these States; for that purpose obstructing the Laws for Naturalization of Foreigners; refusing to pass others to encourage their migrations hither, and raising the conditions of new Appropriations of Lands.
— from the Declaration of Independence, July 4, 1776
Why not return to the open-borders regime that prevailed in the United States from its independence until the end of the 19th century?
I here put aside concerns with national security and deal exclusively with an economic objection to increased immigration.
Given that largely unrestricted immigration into the U.S. throughout the 18th and 19th centuries is widely, and correctly, understood to have been a great benefit to Americans, one plausible economic reason for today’s restrictions is that America is now less able to absorb immigrants because it is more crowded. Even the generally pro-immigration Economist says  that America “has become more crowded, of course.”
But has America really become more crowded? Is America today truly less able than in the past to absorb immigrants? I think not.
First consider indoor living space per person in the U.S. Even since 1950 it has about tripled (with the square footage of the average single-family home rising and the number of people in the average single family falling).
Consider also other amenities, such as medical care, education, and resource availability. These things are generally more available today than in the past.
A couple of years ago  I dug through Census bureau data (and Julian Simon’s book, The Ultimate Resource 2 ) and found data on how much better able America is today to absorb immigrants than it was in 1920 (the last census year before America’s first major step toward restricting immigration). Consider that, compared to each person in 1920, each person in the United States today has available:
– ten times more miles of paved roads
– almost twice as many physicians (the quality of which, of course, is incalculably higher)
– fifty percent more dentists (also of incalculably greater skill)
– three times as many teachers
– close to six times as many police officers
– about twice as many firefighters
– the production of about two and a half times more cubic feet of lumber (as the size of lumber stands in the U.S. also increases).
And, perhaps most importantly, each American worker today has about 8.5 times more capital equipment to work with.
The American economy is today far more able than in the past economically to “absorb immigrants.”