At one point, they had Howard Stern, Don Imus, and Wolfman Jack. But the fact of the matter was that radio audiences had already started to migrate to FM, and it didn't matter what companies did on AM anymore. That was the case in NY, DC, LA, and ultimately everywhere.
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.
In the spring 1973 book
WABC was number 1 with an 8.6
KHJ had a comeback after the failed album cut experience and was also number one with a 6.3
WLS had a 7.3 and WCFL had a 7.2 and were number two and three behind WGN
WFIL had a 9.5 and was number two
KILT had a 10.6 and was number one
KJR had a 12.8 and was number one
WDGY had a 9.9 and KDWB had an 8.8, number two and three behind WCCO
KCBQ had a 10.5 and was number one.
WHBQ had a 13.6 behind only WDIA and the other AM top 40 WMPS had an 8.3
WMAK had a 20.9 and was number one
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
I don’t think the failure of WNBC was because of FM