I think about this issue a lot too, and while I think everyone calling out that these feelings are cyclical, and choosing a decade of that persons own life means that it's just nostalgia has validity, but I also think that, there is probably truth to the idea that our cultural peak is behind us. Some Cultural historians would probably put it some time in the 17 and 1800's when we were building cathedrals and making classical music.

I've become something of a cultural conservative, thinking that most significant cultural lessons, the ways to live the good life can be learned by looking behind us.

There are probably multiple peaks though, and I think a peak in the late 20th century is one of them. I think the 20th century in and of itself will probably always be considered a peak, or significant and the reason is the invention of the motion picture and the way motion picture technology evolved over those 100 years. Our image of ourself as a society will never have quite the evolution you get from having like low fps black and white imagery early in the 20th century, to basically 4k sci-fi like the Matrix in 1999.

But I think the like Stranger Things running around on bicycles, getting to experience the birth of modern technology, sort of a combination of maximal freedom and maximal aesthetic experience, shared cultural touchstones and while not yet introducing social media and post-9/11 anxiety is a signifcant cultural peak.