> In decades past the richest and the poorest people don't remain in that part of the income distribution very long; this is especially true in the US.

I would say this is especially not true in the US where social mobility is notoriously bad.

"Several large studies of mobility in developed countries in recent years have found the US among the lowest in mobility." [1]

[1] https://en.wikipedia.org/wiki/Socioeconomic_mobility_in_the_...