I’m on Windseeker and it’s not even remotely dead.
There will be people devoting time to the first week of a new expansion, and there are people who played Wrath strictly to pass time until DF was released. Even so, Wrath Classic will be fine. Ulduar is releasing soon, and lots of the folks who have Naxx/OS/EoE on farm will be checking out retail until Ulduar drops.
Don’t Chicken Little this kind of thing.