You’ve been reading the forums too long if you think they lost 6 million subs during Cata. It was more like 2-3 million. And an argument could be made that still having 9 million+ subs 8 years into an mmo at a time when a bunch of other mmos were releasing, along with other massively successful games is still a decent achievement. One any gaming company would kill for.
The point that subs dropped off a cliff was WoD. Started at 10 million and dropped to like 4. And shockingly they never released official sub numbers since.