Just mention another obvious issue:
As the years go by, there are more and more artists, albums, and songs that get bumped down into “oldies/classic rock” status. But there are still only 24 hours in a day in which to broadcast those artists/songs, and only so many songs that can be played in that time. So think of the “oldies/classic rock” playlist as your house’s basement, and the artists and songs as old furniture and toys/dishes/clothes/whatever. There’s only so much room down there. As more and more and more artists and songs get reclassified as “oldies” or “classic rock”, your “basement” fills up. Eventually, you run out of room for more stuff.
Ultimately, in order to store new stuff in the basement, you have to make the decision to remove and discard some of the stuff that is already there. So you look around and see that big old living room set that used to belong to your grandmother. It’s beautiful, it’s comfortable, and it’s in great condition … but it’s taking up a whole lot of space. Think of that living room set as a formerly popular artist with a large catalog of hits. You’d like to keep the whole set, but you have to get rid of at least part of it to make room for some newer stuff. So, you reluctantly drag the sofa (representing the bulk of the artist’s hits, but the “lesser” ones) out of the basement and sell it or give it away, but you keep the love seat and the recliner (representing only the artist’s most popular work). Some time later, to free up more space, you dispose of the love seat, leaving only the recliner: the one or two biggest hits. And finally, even that has to go.
Where the basement represents “space”, a radio playlist represents “time”, and in this case they’re the same. You have to get rid of old stuff to make room for the newer stuff. A decision to play one song is also a decision to not play another.
Several years ago, my local classic rock station tried something new (for them). They greatly expanded their playlist, and made the promise to listeners, “You won’t hear the same song twice in a day!” They actually went above and beyond that promise. I listened to that station 8 hours a day at work, and I’m pretty certain that I didn’t hear the same song twice during the first two weeks after the format change.
But that experiment lasted about a month, and then they switched back to their old format. I can see why. When your playlist is “everything” (or almost everything), you’re going to inevitably end up having long stretches in which one listener or another is not going to hear anything they really like. While those streaks are going to be different from one listener to the next, collectively you’re going to have a bunch of listeners changing the dial because, “Man, they never play anything good any more!”.