Players hated BFA, players hate SL, for different reasons that are seemingly contradictory with why they hated BFA.
If, indeed, Blizz is going to hold back 9.1 to give BCC a clear launch, that implies more people reliably are playing Classic than SL, especially since Naxx and SL launched basically on top of each other.
So what would a scenario where Classic is "beating" retail mean for 10.0? Nothing? More wild penduluming between WoD and Legion style expansions? More Classic returning to Retail?