WoD did spike though. It went back to 10 million. It just couldn't hold them because there was no content for people who didn't want to raid. Legion seems to be addressing that. Whether or not it will work is up in the air but We could see another huge spike again. And if Legion is actually good they might actually hold on to more of them.
We will never know though since they aren't giving out sub numbers anymore after this last quarter.