Probably a lot more than 50% of the population has ancestors who were in the US in 1790. There was a huge amount of immigration in the 19th century and early years of the 20th century, but a lot of children and grandchildren of German, Irish, Italian, Slavic, Jewish, Scandinavian, and other immigrants married people who had ancestors here in 1790. Most black Americans had ancestors in the US in 1790.
almost all immigrants assimilated quickly....