I continue to struggle to understand the attention given to relative measures of well-being compared to absolute measures of well-being.

If everyone's wealth doubles tomorrow, this chart will not change at all.

There is an acknowledgement in paragraph 8 that "middle class ... median income, adjusted for household size, has risen over the long haul, increasing 34% since 1970" but this good news is necessarily moderated by the gloomy observation that middle class median income "has not kept pace with upper-income households."

Who cares? Would any members of the middle class be better off if all those upper-income households vanished?

The "34%" is important as it is adjusted for inflation and household size. We often see factoids claiming that things were better back then, but even if this number were 0% it would show that things were not better. Things are better now. And income measures ignore the much greater variety of things we can spend income on today, many of which are less expensive and higher quality as well, from vegetables to automobile safety (updated link to video).

Controlling for household size is important because "average household size in the United States decreased from 3.2 persons in 1970 to 2.5 persons in 2015, a drop of 21%" according to the methodology.