> has among the lowest social mobility of developed nations

The US also already has among the highest median income of any developed nation, and nearly the highest median disposable income. The US also has a higher median household net worth than either Germany or Sweden. That would inherently make it more difficult for the US to keep pushing the boulder up the hill.

Further, in the US more people are moving up out of the middle class and into the higher income brackets, than are moving down and out of it. That has been true for 40 years running now. Given the immense scale, income and wealth involved in that result, it's a staggering accomplishment.