If the bulk of your tax bill comes from special capital gains rates on the fraction of your net-worth-increase that you are ignominiously forced to admit constitutes capital gains, you're an elite. If the bulk of your tax bill comes from a higher income tax tier, you're middle class. If the bulk of your tax bill comes from a lower income tax tier, you're correspondingly lower class.
Needless to say, the principle that taxes reveal true power does not lead me to agree with your premise that 60% of the middle class has disappeared into the upper class.
https://www.brookings.edu/research/squeezing-the-middle-clas...
> 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
The "elite overproduction" framework begs you to not view the middle class as a group of people who have achieved a balance where they do valuable work and are rewarded with nice things, but as a group of scheming do-nothings who pose a burden to society as they foolishly aspire to elite-hood and repeatedly fail. It's a laser-guided propaganda missile aimed directly at a fault line in populist movements that have been gaining steam recently.
Given that people like me have only ever participated in the economy during a time when the middle class was shrinking, what are we to do? Most of us agree, if our future is a world of a few 'haves' and many 'have nots', priority one is to become a 'have'. Even from the most moral stance imaginable, a 'have not' has no ability to help others. Hence, too many of us try to get into the elite. To aim for the middle class is to aim for grinding poverty, and why would anyone?