Once upon a time, the top 40 was determined largely by sales of 45 singles (records) to 12-13 year old girls. Back then, teens grew up and didn't attempt to be cool into their 40s, but became adults, although they might hear the occasional pop song they enjoyed. To an extent, it's still that way, but "adults" are more likely to have another identity as a pretentious cool person. (Ask teens which they prefer: adults who act like adults or people in their 40s who want to hang out with teens and "fit right in." Especially if the adults are their parents! Guess what?)
I could write a book, but I'll just confine myself to one more train of thought that could easily be extended: golden oldies. If you're 25 or younger, have you ever heard a song by the 60s band Free except "All Right Now"? I doubt it...at least, not on the radio. Classic bands that sold millions of records are reduced to their one or two (possibly 3 or 4 if you're talking about the Stones or the Beatles) biggest selling singles. If you get the chance, find someone who has the two-volume version of Free's biggest hits (the 1-volume version leaves off "I'll Be Creepin'," just as Aretha Franklin's "Golden Decade" suffers from the absence of "Chain of Fools"; go figger) and check out "I'll Be Creepin'" and "The Stealer." You may never have heard anything by Free except "All Right Now," but U2's The Edge has. It shows in his playing, especially when he frets up the neck on some strings, while playing open strings at the same time.
Just as Led Zep was patterned after the original Jeff Beck Group (with singer Rod Stewart and wailin' bassist Ron Woods), Free was also vocal (Paul Rodgers, who went on to found Bad Co), guitar, bass, drums (also went to Bad Co). Okay, occasionally piano on records, but basically 4-man bands all.
Judging by what "music" is coming out today, I think the 12-13 year old girls did a better job...and diversity has always had a place when it hit the spot. If you don't care for, say, "La Bamba," we probably wouldn't have a lot to talk about. Los Lobos was good, but Richie Valens (Valenzuela: we weren't THAT diverse!) nailed it.
My thoughts only. You can have different thoughts; that's okay.