The reason this is important isn’t for the numbers, it’s for the fact that there is far more interest in Classic than in the current version of the game. This should be a pretty clear indication that the current DIRECTION of the game is not sustainable. People want gameplay that is more like the original game, and either Blizzard provides that, or someone else will.
IMO, the smartest thing they could do right now is put together a team of their best people to start re-imagining each WoW expansion in terms of the classic game mechanics. How could you retell the storyline of Mists of Pandaria but keep game mechanics that are more in-line with vanilla?
Even the dreaded WOD story could be salvaged if they could find a way to not burden it with dumbed down systems and isolating garrisons.