To politicians of old, “the American dream” was a supremely optimistic rhetorical device, albeit one that often obscured the economic and racial barriers that made achieving it impossible for many.

Nonsense. People didn't move here because they heard a politician say they can get rich. They moved here because a friend of family member either moved here or knows someone who moved here and showed them how much money they can earn compared to their home country.

If it weren't true, people would have stopped moving here long ago because word would have spread that it's all bullshit.

But it's not (yet) bullshit so they continue moving here in droves.