There's been plenty like the 30s since. Old time radio held on for years and was celebrated on some AMs even into the 80s.
But not on top rated music stations like WOGL or even WCAU-FM in the 70s. Old Time Radio basically disappeared when the big stars transitioned to TV in the 50s. That's what opened the door for local DJs. Sure some AM stations continued to air radio drama late Sunday nights into the 80s as you say. And actually stations like WXPN play some progressive rock today as though it's still the 70s. In fact, WXPN did a huge re-enactment of Woodstock last August. That's where people can still hear music either from that era or influences by that era. The music is available, just as the music of the 30s & 40s continued to be played until recently.
Meanwhile, terrestrial music radio isn't inventing new formats and its audience is splintering, just as musical tastes did in the 90s and after.
Keep in mind that radio companies aren't in the music business, so it's up to the music business to create the music. If it's popular enough, radio will invent formats built around that music. That's really how rock expanded in the 60s, and later how smooth jazz came about. It begins with the music. Right now, radio stations are mixing in a lot of new musical genres into existing formats because music taste isn't as focused on specific genres as it used to be. But there are some types of music that are attracting mass audiences, and those are the ones radio is playing.