How did Country stations have weekly surveys that were 30 or more songs long, if they weren't playing almost all currents? When would they have time to play a 30-40 song current chart if they were playing much library material? Same with R&B stations. Their current charts were 30-40 songs long too!
In the 60's and 70's, current songs were on average less than 3 minutes long. So if a station had 15 to 18 minutes of ads, news and other stuff, that meant about 14 to 16 songs an hour.
My own first Top 40 in 1964 played 18 to 20 songs an hour, and never less.
Prior to that, I worked at an R&B station where they also played over 15 songs an hour on average.
So if you had 40 hit songs, and played the hot ones every 2 hours to 2:30, and the bottom 10 every four to five hours, you could easily play 10 to 12 currents and "hitbounds" an hour as well as 3, 4 or 5 gold songs (we had not invented the term "recurrent" yet and thought a song had to be "chilled" for a while before coming back as an oldie).
WABC was famous or maybe infamous for having only a 14 song chart, with a handful of "bubbling under" titles. But WABC rotations were so fast, #1 played nearly every hour and other hits every 90-120 minutes. How would WABC have time to play much library material?
The #1 song caused a light to flash every 90 minutes in the studio at one point. They played it roughly 100 times a week. And while the list was short, it was a lot more than 14 songs.
I'll stand by my statement again. Album Rock stations played a good deal of library material. Top 40, Country and Urban stations did not.
Back in the 60's and 70's before Urban Cowboy attitudes took over, there was very deep library play on country. If you listened to the Opry, the members would sing a new song and several old ones. Country stations did not have as regimented a Top 40 list, and they played lots of "superstars" as well as new songs.
Top 40 was conceived to cover today's hits. So of course they played little gold. But R&B stations long before the "urban" term was invented played lots of big name artist gold always... right back to what WDIA and the earliest stations of the type played.
At WJMO where I worked, the studio had a record library that covered a big part of a whole wall, and included lots of things that could only be played in certain shifts or days of the week... early dayparting.
And that's why Classic Rock stations can go back further in their playlists than other formats. It's because their listeners don't consider those artists unacceptably old. Growing up, they heard The Who, The Stones and Led Zeppelin as frequently as they heard contemporary rock artists.
It's more about the quantity and quality of new releases. AOR stations in the 70's focused on "superstars" and that is what Burkhart-Abrams called the format. In fact, there were places in the clock at some of them that allowed for "any approved song by any of these star artists".