If they didn't have WoW as cash-cow they could not have been able to afford beta'ing content for a year, cutting out units.
The sales figures can't be steady, the entire focus is on competetive PvP, a toy to wave around at tourneys the casual crowd isn't touching it, twitch stream numbers have steadily declined over the last 2 years. They are just releasing content for SC2 out of obligation above all else. Starcraft 2 released in 2010, HotS 3 years later, Void most likely 3 years after HotS.
