Wow doesn't necessarily require more single player content, however more things to do outside of instances might help the increasing number of players who don't just use the map as way to get to instances.
The reason there's so few people in the game right now is because very few people actually aim for the end game achievements. Last time I checked something like 4% of players had curve for Jailer (never mind CE) and like 20% had KSM. While it's easy to think everyone has these things, that's only true in the bubble of people who have climbed the ladder that far.
We need solo and group outdoor content to help satisfy those who aren't instance junkies.