What are they doing instead? WoW will die eventually, there's no way around it. And I don't believe for a second they're making as much money with that shitty cardgame as they're making with WoW, even in its declining state.
I don't know how you can think Hearthstone is not making hand over fists.
And really, that was out answer. What is the WoW killer?
League of legends, Minecraft, and hearthstone.
Expecting an mmo within the tiny box of what we consider a mmo to be the successor to WoW was a huge mistake. WoW itself was a flash in the pan. Like the Wii, it tapped into a market. It exploded, and while may have expanded the overall marketplace. it did not do so on a way in which represented new users branching out to other games.
WoW's success is much like LoL's. its success fed into more success. Massive multiplayer games like this feed off large playerbases. Think about every MMO you played, and when picking your server. the most populated server is the most popular server. meaning, NEW players will inherently FLOCK to the most populated one. The same applies for the games themselves. And this is why no MMO can break WoW. those players are not leaving WoW and WoW has become a cultural phenomenon, so even new players are still more inclined to go check IT out, over new IPS.
LoL and minecraft's success is explained in the exact same way. And in this point, as new genre/markets they were allowed to be break outs.
Even trying to replace WoW would be a huge mistake on Blizzards part. the people playing it will be totally happy with it just being updated.