If “decline” means it was dying, then the very fact it got popular means it was NOT in decline.
By the same token, just because it’s popular doesn’t mean it’s bad.
“Decline” also means loss of quality. C&W took a hit quality-wise after UC to make it more appealing to the masses. See post #50 from OttawaFreeper.
Likewise, SNF pushed the Bee Gees homogenized disco sound to the forefront, to the detriment of funk and R&B.