Before WWII, middle-class married women were strongly discouraged from working for pay outside the home. If their husbands could provide, "respectable" women were expected to stay home as homemakers.
One could argue the opposite: that the mass entry of women into the paid workforce expanded the labor supply, contributing to wage stagnation and, eventually, the erosion of the middle class. But that wasn’t the only cause. Globalization, declining unions, automation, and regressive taxes were also factors.
> 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.
See page 3 on https://www.nobelprize.org/uploads/2023/12/goldin-lecture-sl... for an illustration.