So I saw an article with this chart in it. It shows that since 2010 when WoW was at it's peak, amount of money people spend on games and PC games has declined each subsequent year. To me this clearly shows that WoW's decline isn't some crazy "Blizzard is awful, Ghostcrawler is an idiot" but is largely a reflection on the changing preferences of consumers.
I don't know how, in a market like that, anyone would expect WoWs subs to not decline noticeably.