If you go to wowhead they have the server data up for Classic. There are like 4 million people playing classic. No way retail has even close to that. I think its safe to say that the wow killer was wow.
My guild has been mostly quiet but it was bound to happen even without classic. We got AOTC, we dont mythic raid. People bring alts to heroic raids now. We pug a lot. People always applying though.
Im not playing classic with them though. Most of them actually quit playing classic already.
I’ve noticed zones feeling less full because flying exists, but the amount of players available for content doesn’t seem to have decreased. If anything, there are even more people leveling retail alts and doing legacy raid mog runs now that Classic brought subs back.