in the 1940s this situation still
applied. But as people moved off the farm and into the city after WW2,
the need for clear channel stations disappeared. Stations which once
broadcast news and entertainment to farms refocused on their local
markets. And then came TV in the 50s which took away AM's traditional
news and entertainment schedule, and then FM in the late 60s which took
away the music market,

I agree with your analysis in general, Tommy, but your timeline is a little
off. If it weren't for clear-channel AM stations in the late '60s and early
'70s, I never would have heard any rock or pop music. WLS, KAAY, CKLW,
WCFL, WIND... dose were da days.