> The article explicitly removes decline in religion as an explanation for this particular bout of unhappiness.

It tried to, at least, but I'm not sure it succeeded. The growing secularization up to 2020 follows the long-term trend towards unhappiness and peak secularization and peak unhappiness line up too. Happiness has even started to improve in line with the growing return to religiosity that has occurred most recently. The data it presents as supposedly dismissing religion actually makes a reasonable case for religion.

Of course, the reality is that there never one reason. Americans are sad for millions of different reasons. The idea that if we fix that one thing all will become right with the world is pure fantasy.