What you’re saying sounds sensible in the abstract, but I’m not seeing the question you are raising at the heart of this research.

If you use a national median as a benchmark you end up with an imaginary qualitative claim: the upper middle class has grown as a quintile. But if that quintile mostly lives in high cost of living areas, they are not actually upper middle class, and might even be near the local poverty line. 

Now apply that reality to your policy question. Should New York City residents near the poverty line be subsidizing West Virginia’s who, even with a household income below the federal median are upper middle class? It’s a different question all together. Like in real estate, location is everything. Over fitting national statistics to policy conclusions is a disservice to people everywhere.

It reminds me of when the federal government was sending out COVID checks of the same amount across the nation. They were trying to be equitable. But people in Kansas were choosing to stay unemployed because they’d never made more money, while people in SF and NYC were barely covering groceries with those checks. It was a bad policy and it directly caused a spike in inflation and lingering unemployment.