What can they do?. Nothing.
Im sorry, but this negative argument over Blizzard losing subs, and the impending doom it represents is tiresome and old and needs to be put to rest.
This game is old, real old in the scheme of things. We are coming up on a decade very soon since its released, Blizzards notion of attaining this mythical golden pot at the end of the rainbow that is "new players" really needs to take a back seat to retain the current playerbase still hanging on.
The majority of people outside the WoW loop look at its aging graphics engine, its slightly hostile established player base when dealing with new players, the intense levelling curve, skill and gearing curve and a myriad of other issues that come with an established MMO like WoW and either quit after level 40 odd or just dont even bother.
ESO comes out this year, along with a flood of other great single player games, its only natural attrition over years of players burning themselves out and leaving, or simply just finding another game, its not the end of the world and i personally am sick and tired of people seeing the loss of subs as such. Be thankful that we have what we have, one day soon within the next half decade it will dwindle down to less than 2mill and will still hopefully be one of the bigger heavy hitters on the market.
I love this game believe me, and still play it daily. But i guess ive played it long enough to recognize the signs and where its inevitably heading