Earlier quoted context omitted.
That was almost entirely due to America rebuilding Europe and every other major industrial power knocked off their perch for a couple of decades. It had nothing to do with immigration. Additionally Americans were still having lots of kids at that point so the population was growing without immigratiom.
Those European countries also developed fast without much immigration (including Switzerland and the Nordics, which were not affected much by the war). It may have nothing to do with immigration, but you certainly haven't proved that. Edit: Currently the poorest parts of the US population are stagnating at best, at a time when rates of both legal and illegal immigration are quite high. It is hard to make the argument…
France invited large amounts of immigrants from Africa to help rebuild so it’s not like they didn’t see immigration.
I think there are two main reasons that the poorest parts of the population are stagnating. One is poverty traps, and especially multi generational poverty traps are very hard to escape. Another is geographic location. Americans dont move as much as they used to, all around the world poorer people move to areas with more jobs and opportunity but today mostly richer people move and poorer people stay put in areas that are impoverished.
Immigrants largely don’t come with a set geography or generational poverty weighing them down which is why their kids do so well.