I liked the format WEBR signed on with a soft oldies/standards mix. After all WKBW played some Mom & Dad tunes mixed with the Beatles and Stones. We grew up listening to a very interesting mix that's not availabe anywhere else.
Top 40, from the mid-50's through the 60's, was a broad format because FM had not found its place yet (forced by non-duplication rules after nearly 30 years of failure or scant revenue).
Look at markets like Cleveland, Rochester, Buffalo, Pittsburgh and the like. Maybe 6 or 7 decent AMs that were fulltime and covered at least most of the market. Some were MOR, one might have been country in a market or two, one might have been R&B and a couple were Top 40.
So the Top 40 stations tried to be all things for all young people, including younger adults. Even then, aside from some pimple cream ads and the like, there was not enough teen money for a station to live off that group. So stations played a range from Dean Martin to Iron Butterfly.
When FMs were forced to find a different format, some picked a variety of Top 40. So we got album rock, Top 40 without the MOR / adult songs, "chicken rock" which became AC and lots of other formats that splintered the traditional music formats of the prior 15 or so years.
But from the mid-50's through the 60's, those of us at the younger end of the demos knew that our markets had two or three Top 40 stations most of the time. I was in Cleveland through 1962, and nearly always had three Top 40 stations to choose from... so when Domenico Modugno or Sinatra or Nat "King" Cole came on, I switched to another station.
My point is that what you call an "interesting mix" did not make anyone enormously happy. The older listeners who were not into the very old MOR did not like the rock and bubblegum and The Twist and so on. And the younger ones, like me, did not like the stuff that sounded like those old crooners.