If two million people left and two million new players joined in the same quarter, the subscriber graph would stay steady. That is definitely not a "plateau" which is why looking at subscriber numbers and deriving any kind of conclusion is faulty. We simply have no idea how these numbers were derived without knowing either how many players the game lost or how many new players the game gained.
Moreover, two years of roughly the same subscription numbers is a pretty good indication of market saturation. You see this same phenomena in other subscription services like cable television where certain providers will simply stop marketing in areas once they've reached the theoretical maximum number of subscribers.