Phone games are pretty popular these days. I don't think their numbers help since WoW is not big compared to Hearthstone or other games.The numbers are stupidly high and says something about how big Blizzard is. Millions upon millions have bought wow or other Blizzard games, thats not "normal". Its quite a feat. Now - most people treat games like this: Buy game, plays it. When bored, stop playing it. Its not 2008 anymore, people can pick and choose from a pletoria of good games that you can get hold of in no time. Heck, many of them are even free!
Back in the "old days" people had wow as the MMORPG to play(think wotlk days). There was no other MMORPG that could compete at all. It was a different time and the options wasnt that big, especially when it came to an online game of this magnitude. To keep hold of 10+ million subs for over 16+ years is impossible.
What im trying to say is: People shift and move around with games all the time. People dont stay subbed throughout a xpac anymore, cause why would they? New wow xpac/patch comes, people sub and play for a while. Then stop and move on to other things. As it should be. You dont have to stay subbed all the time to wow to get fun out of it, and I doubt even Blizzard wants you to sub and play all the time. Theres even a ingame tip were it says you should take breaks lol.
Could wow do better right now? Sure, it always can. Is it dead, buried and forgotten? Not by a long shot and when 10.0 comes around people will return, play it for a while and have fun with it, and unsub again. Nothing wrong with that.