"Economist Nathan Smith has long been a supporter of the U.S. opening its borders. He's argued that the American polity would "endure and flourish" if it elected to lay out 2,000 miles of welcome mats rather than building a Trumpian wall. He's claimed that unencumbered movement of peoples from land to land would result in worldwide GDP doubling, though not without some serious social destabilization. Still, he predicted the U.S. could handle an influx of 150-200 million immigrants over a span of several decades.
(...)"Smith wrote again on the topic of open borders a couple weeks ago in a piece that plays out like a giant thought experiment. Smith's hypothetical situation: If all borders were opened and 1 billion people immigrated to the United States over the course of 50 years, could the country maintain its "political character and structure"? Smith thinks not.
(...)"What's most interesting is how Smith conjures a scenario out of which American constitutional democracy becomes so destabilized that it collapses beneath its own weight. We'd be looking at a new world order and an American polity unrecognizable compared to the present. For many people, that might sound like a reason to scrap a 100 percent open-border policy. Smith is not one of those people. He's got a bone to scrap with American politics and wouldn't mind it turning into collateral damage amidst the rush of a brave new society..." Read the article: Thought Experiment: What If the U.S. Had 100% Open Borders? Over a year ago by Robert Montenegro, Big Think
Related article: If People Could Immigrate Anywhere, Would Poverty Be Eliminated? -- Some economists are pushing for "open borders" by Shaun Raviv, Apr 26, 2013 The Atlantic