1960s is only 10 years so if something went on in the public mind day on and day off for 5 years (like COVID did for 3 years), I think it's a good candidate for the decade-defining theme

The lingering effects might very well last longer than the actually event itself though. Hell, the effects of LSD last much much longer than the drug itself lasts in the body. So to me, you're harping on the wrong thing. Sure, maybe the movement lasted a short time, but the mindset is still out there

There’s a related occurrence of the same principle — we have had movies/stories about the wild west for longer than the archetype frontier period existed.