Perceived wealth inequality. The US has less wealth inequality because most of it is already wealthy. The shrinking middle class is shrinking because most of them are moving to the upper middle class.
How does that support the hypothesis that the middle class is shrinking because incomes are rising?
Also, here's a more recent source (August 2020) that's actually an academic paper -> https://www.brookings.edu/wp-content/uploads/2020/08/Squeezi...
> The analyses presented here confirm the broadly accepted picture of rising income inequality and slowing income growth for middle-class Americans. But a few additional points are worth drawing out. First, while the benefits of economic growth have not accrued equally, they have not gone solely to the top 1%. The upper middle class has grown. Second, the main reason for the shrinking of the middle class (defined in absolute terms) is the increase in the number of people with higher incomes.
Historically, the middle class has been shrinking because the upper class has been rising. Insofar as that hasn't been the case since 2000, it's not because of the mere existence of billionaires, it's because of a specific type of financial instrument (in one case), and a one-in-a-century global pandemic (in another case). The mere existence of Bill Gates et al did not cause either the credit default swap crisis or COVID-19.
Second of all, Piketty's argument is more that capital's share of growth will necessarily outpace labor's share of growth (r > g).
Third of all, Piketty isn't gospel. There have been a number of rebuttals published since his findings that make fairly strong refutations.
The IMF studied empirical evidence to see if it matches up with Piketty/Saez/Zucman's theoretical models, and was unable to validate their finding[1].
Further studies showed that r > g almost entirely goes away when you exclude land/housing appreciation, mostly attributable to restrictive zoning regulations [2].
Auten & Splinter found that Piketty failed to account for existing taxes and transfers. When you do that, the perceived growth in inequality goes away almost entirely[3][5].
[1] https://www.imf.org/external/pubs/ft/wp/2016/wp16160.pdf
[2] https://core.ac.uk/download/pdf/35310497.pdf
[3] http://davidsplinter.com/AutenSplinter-Tax_Data_and_Inequali...
[4] https://www.nytimes.com/interactive/2015/01/25/upshot/shrink...
[5] https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3546668