Huh, TIL that JFK was blamed for killing the hat market. Literally never heard this until you posted that!
I’ve been into hats my whole life, so I’ve learned a bit about them, historically and anecdotally. Thing is, you can SEE the slow, gradual decline in hat wearing in documentary films and popular entertainment with your own eyes. Go back before the Great Depression, and you’ll almost never see a man without some kind of hat, with many styles directly linked to status or profession.
You’ll see more hatless heads just prior to WW2, but not many. After the war, though? Most hats didn’t fit the styles favored by the youth, and by the mid-1950s, almost all of the hat-wearing characters in contemporary entertainment were parental figures or men of economic & political power. Get into the early 60s, even many of the main characters had stopped wearing hats.
Even with entertainment aimed at younger audiences where the characters themselves might not be hat wearing, there were signs of the decline. Looking at the set designs, hat & coat check rooms disappeared almost completely, and in the background, hat & coatracks had started to dwindle as well.