So yes, the homogeneity began 70 years ago. What you're hearing now is nothing new. The only reason you know about it is because of streaming, where you can sit at home and listen to thousands of radio stations around the country. But what they're doing was being done in the 60s. If you go to American Radio History, you can see the R&R charts from the period and see how many stations were playing the exact same songs all across the country. That's why we all know those songs. But it's more about the music business (which also consolidated in the 80s and 90s) and not as much about radio.
In support of your position, we can go back to the way the record labels supported the "Tip Sheets" going back to Gavin who started in the late 50's and was then followed by an explosion of the genre with FMQB, Hamilton, Radio & Records and a whole bunch more in the 70's.
There were rock sheets, Top 40 ones, Country editions and even college station publications. Almost all were full of record company ads for the new releases that they were working (the exception was one college radio sheet that felt that selling ads was a sell-out).
If you were at a key major market station that reported to one or more of the tip sheets, you could count on lots of calls and visits from "record ducks" on the day or two before weekly reporting, all saying "play my record" over and over.
And we looked at those sheets in great detail, knowing that it was a great way to find songs that were moving that we needed to consider strongly. It was also a way to make sure that we had not missed a song that was breaking big at a station we followed closely and respected.
And thanks for mentioning
www.worldradiohistory.com!
Most of the tip sheets are located in the sub-heading of "Radio and Music" part way down
https://worldradiohistory.com/index.htm
For those unfamiliar, at most stations magazines like Billboard, Cash Box and Record World were either not received or only viewed for the artist news and not the charts. Tip Sheets were based on weekly reports from as many as a hundred or more stations in a format showing the adds, drops, powers, fast risers and the like. That data was tabulated to show the big moves of the week.