“Open borders” was pretty much standard across the world prior to World War 1. These tightly controlled immigration policies are, historically speaking, incredibly new.

I think it’s self evident that the U.S. benefited greatly from its mass immigration inflows in the 19th and 20th centuries.

It's a different world now

Your statement has no basis whatsoever in reality. The US, for example, had a four-decade moratorium on immigration beginning in 1924. Mass immigration flows appeared at various times and places in the past (often accompanied by bloodshed and suffering), but it's highly incorrect to imagine that 21st-century 1st world demographic shifts are some sort of historical norm.