While you are correct as far as you go, the replaying of hits basically 20 years (or more) after they were hits started during the 1970s. In fact, during the 1960s, there was no mass desire to play 1940s hits again; nor did that desire come into play during the 1950s to replay the hits of the 1930s. The fact that the 20-year nostalgia boom began in the 1970s suggests to me that the nostalgia booms from that decade on were, at least in part, manufactured by the music industry to encourage people to purchase more of its older products.
I don’t necessarily think you're seeing the full picture. The notion that there was no mass desire to play the 40’s during the 60’s wouldn’t seem to be supported by the data. I'll grant you that I wasn’t around in the 60’s and finding format data from that time is a bit tough for a variety of reasons, but MOR existed. Every station that existed in the 60’s wasn’t playing rock and popular music. MOR was still in the money demo until at least 1980. My grandfathers were born in 1914 and 1924, and they listened to MOR and big band music almost their entire lives. The older grandfather lived to 85 and had access to a satellite standards station until he died. That's a mass desire to play and hear 40’s music. You just didn’t hear it on the stations you liked. That standards station, by the way, ambled on another few years after the older grandfather died, but it didn’t quite make it long enough to survive the other one.
I grew up mostly during the 80’s, and I generally didn’t hear 60’s music on my stations. When my favorite Top-40/CHR in junior high “sifted through its old record collection for more variety,” I started spending more time with the competitor across town.