LOL. Yeah, there's still an incentive and drive to assimilate.
The biggest migrant wave started in the 19th century when the circle of racial acceptability grew from Anglosaxon to encompass the other races of Europe. The continued growth of that circle and the shedding of ignorance about other races isn't a retcon and is wholly precedential.That's a retcon adaptation borne from the 20th century, contemporaneous with the 1965 Immigration Act. Has little to do with what America's historical character had been 200-300 years prior.