Sure, but since 1967, upper class has risen. The Great Recession is a confounding variable.

In a causal system the Great Recession is not a confounding variable, it's an inflection point. As will be the recession triggered by covid.

It is definitely a confounding variable in the context of the question at hand (posed by the GP commenter): why inequality is a bad thing, and whether it is directly responsible for the "shrinking middle class" going to the "lower class" instead of the "upper class".

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.