It's getting to the point where I search "K-shaped" and "Cohort" in these kinds of articles before I even read them. I'm not even saying these are why, exactly, but failure to wrestle with the intellectual effort of rejecting that as a hypothesis is a frustrating omission.

The article shows that the decrease in happiness is across the board, so even the wealthy cohort (the upper arm of the K-shaped chart) is unhappy.