Here are two reasons for the decline of radio:
1) MONEY. Chasing a finite number of broadcast channels: formats that appeal to a minority can't generate the dough needed to stay on the air. Original owner(s) find MONEY too attractive, so they sell out.
2) Consultants - Once MONEY owns the channel, they need advice about how to maximize the value of that channel; consultants tell them how to do it
(This rule applies across the spectrum - or, why does your local public station have such a limited play list; see also, Item 1,above)
I started listening seriously to radio in the mid-50s in the Bay Area, just as the major networks were declining, and before Item 1
started to dominate the air waves. A golden age? It was for me.