All this implies really, is that raiding was not the reason a significant chunk of those people left.
That really isn't the point though. The point is that there should be, in theory, something that a player considers worth doing, and isn't immediately within reach.
Whether Blizzard doesn't currently have *that* formula down (but arguably did at one point), or there's simply a lot of players that probably shouldn't be playing an MMO to begin with (equally likely), removing all challenging content is probably the worst solution that could be arrived at.