All the things you just said suggest a high retention rate, with people returning to the new game with each new expansion, rather than new players arriving, being disappointed with the quality and then leaving. You may have anecdotal evidence of people leaving the game because they're unhappy, but I have anecdotal evidence of people staying because they're enjoying themselves.
Now I didn't buy BfA on launch (or at all in fact, a friend decided to buy it for me recently). I initially didn't really have any interest in playing this expansion, and I mostly come back for the community not the game itself. In recent years have become more interested in the narrative of video games, in terms of video game design I think BfA is fine, but when it comes to narrative WoD derailed that solidly with time travel.
I'm not sure what parts of the game are cash grabs either, in terms of content I would say that they have released a solid amount of content with every expansion (especially if you compare TBC/WOTLK with MoP onwards) - the only obvious cash grab (IMO) was WoD as it was playing purely on Nostalgia of WC1, 2 and 3 characters.