WoW doesn't need to up the player numbers.
As long as there are more than 2-3 million subscribers the game is more than profitable.
As for the player enjoyment: the connected realm play assures everyone that enough players will always be around (both for the AH's as for the content runs).
Blizzard found the Holy Grail of MMO's with connected zones and realm play.
Not all players have realised this yet, but WOW is now independant of players on seperate servers. So the game can even support 200K players per region now.