High refresh rate has to do with reaction times in competitive multi-player gaming.

You absolutely don't need 120 Hz in any single player video game produced between 1978 and 2020.

Games at 15 FPS (even less) used to be perfectly playable and fun. At 30 FPS things are smooth well past giving a damn.

You can't play a game at 15 FPS. It's a moving slideshow. And it was never fun. 30 fps looks pretty bad, and even 60 fps is decently choppy. I'm not sure what your exact point is, but once you try 144hz or higher, you'll never, ever, go back.

15 fps was never perfectly playable.