i'm reading piketty right now and have to say, some overwhelmingly clear and rigorously collected data in this one book alone convincingly contradict this ill-informed statement
First of all, the observation that US's shrinking middle class is attributable to an increasing upper-middle class is objectively true, it's not that controversial [4].
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].
