Calm down. I wasn't writing about a drop-off in BFA. The thread topic is about a drop-off in Classic which is clearly happening. No one but Blizzard will know how many subs are going to which game although if I had to bet the majority of them will be playing both. Also you nor anyone else here has anything more than faith about retention. Faith is fine; we all need some faith. I'm well aware that Blizzard designs to sell expansions and less about keeping people around.
The bottom line is play one, the other, or both depending on your mood. We should want both games to be successful.