> As for the middle class, most of the reason for the decline is people moving into the upper class.

Thats not what I get from the source you provided.

It shows that middle (and lower class) are massively losing income share: Ine 1971, you have 88% of population in middle class or below, with 72% of total income.

81% of population remain in that bracket, but now they only get 51% of total income. That is massive, and also bad for the economy as a trend because rich people spend less of their income.

The conclusion I draw from this is that middle-class (and below) is in decline because the rich "upper-class" is soaking up much of their income.