I think a very large chunk of wow relies on hard content and I think those are the most stable subs. Right now the lions share of wows player base is on area 52, malganis, illidan, tich for horde and stormrage and sargeras for alliance.
Mythic has become the pillar of the game and given blizzards utter reluctance to allow mythic cross server till it is mostly irrelevant to the crowd it was designed for I suspect players moving around mythic servers make up a decent amount of wows profit.
I think final fantasy works with easier levels of content. I think if wow went down that route it wouldnt kill it but it would be a very large blow to it.