Not everywhere, WLS had a 7.9 as late as 1979, second behind only full service WGN.
I would argue AM top 40 was still viable in the early 1970s.
Well, yeah. The early 1970s. But let's go five years-and then ten years- out from every one of those examples:
In the spring 1973 book
WABC was number 1 with an 8.6
Spring 1978, still number 1 with an 8.1.
Spring 1983, no longer in the format.
KHJ had a comeback after the failed album cut experience and was also number one with a 6.3
Spring 1978, tie for 6th with a 3.6.
Spring 1983, no longer in the format.
WLS had a 7.3 and WCFL had a 7.2 and were number two and three behind WGN
Spring 1978, WLS was 3rd with a 7.2 and WCFL was no longer in the format.
Spring 1983, WLS was 8th with a 4.1.
WFIL had a 9.5 and was number two
Spring 1978, fifth with a 5.8.
Spring 1983, no longer in the format.
KILT had a 10.6 and was number one
Spring 1978, fifth with a 6.3.
Spring 1983, no longer in the format.
KJR had a 12.8 and was number one
Spring 1978, fifth with a 6.8.
Spring 1983, no longer in the format.
WDGY had a 9.9 and KDWB had an 8.8, number two and three behind WCCO
Spring 1978, KDWB was fifth with a 5.9 and WDGY was no longer in the format.
Spring 1983, KDWB was no longer in the format.
KCBQ had a 10.5 and was number one.
Spring 1978, fifth with a 5.5
Spring 1983, no longer in the format.
WHBQ had a 13.6 behind only WDIA and the other AM top 40 WMPS had an 8.3
Spring 1978, WHBQ was 4th with a 9.1. WMPS was no longer in the format.
Spring 1983, WHBQ was no longer in the format.
WMAK had a 20.9 and was number one
Spring 1978, tied for eighth with a 4.3.
Spring 1983, no longer in the format.
WKTQ in their first book as a brand new AM top 40 in 1973 had a 15.3 behind only KDKA, beating KQV which had a 6.2
Spring 1978, WKTQ was seventh with a 4.7. KQV was no longer in the format.
Spring 1983, WKTQ was no longer in the format.
I don’t think the failure of WNBC was because of FM
No. The failure of WNBC was because WNBC never figured out what it wanted to be and could never make the case for New Yorkers leaving WABC. And you're not wrong about NBC not being great at radio. KNBR in San Francisco was an also-ran to KSFO until they won the Giants contract.
But in every other market beyond New York and Chicago that you cite, the stations had significant to catastrophic losses of audience in the five years between 1973 and 1978. And they didn't lose that share to other AMs. And given five more years, WABC and WLS were much worse off than they were in '78.