I'm curious as a listener. I've seen some stations which really play it "safe" with their music choices eventually fold, while others do fine with a very conservative playlist (mostly Chrs.) I'm wondering, if songs test well and are overplayed or become repetitive to a group of listeners, if they might tune out. For example, what's preventing"don't stop believing" being played 3-4 times a day on a station? This is pure curiosity and I don't want to ruffle any feathers (though I know how thin skinned some can be.)
Music is tested constantly and adjusted when songs become stale. A good example of that is Fleetwood Mac. Their most popular songs today were relative stiffs when they were new. There are a couple songs off of Rumors that were popular as currents and remain so today, but many of the songs of theirs you hear on classic hits stations didn't even make the top-20 and got most of their play on album stations.
While there's a fine line between popular and burnt to a crisp, people want to hear their favorite songs and most would rather hear a few of them a couple times a day than hear a bunch of songs they don't know or like. Everybody says they want to hear variety, but they mean "all of my favorite songs" when they say that. The listener response to stations that have long playlists and actually play real variety is usually, "God, they suck!"
Something I learned in Marketing 101 in college was "always segment the market." If you're in radio, you're doing that anyway by playing different genres of music. People who don't want to hear the repetition of top-40 stations generally don't listen to top-40. Top-40 is also a format that is driven by cume, not TSL. So, the programmers already know a substantial number of listeners are going to tune out as each day goes on. There are other stations, often in the same cluster, that offer alternatives for the top-40/CHR audience that doesn't want to listen to the same songs all day long.