Sure, but the root cause of that is pretty well known to be the proliferation of credit default swaps that led to a housing bubble that burst in 2008, and not the mere existence of billionaires writ large.

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.