He defines the direction yes, but inertia also matters. Twitter may be small by Facebook standards but it's still one of the largest social medias in the world. If you grab the wheel of a giant ship moving full speed forward in a given direction, you may want to set direction, but it'll take you time to steer it in that direction.

In 5 years Twitter will be absolutely nothing like what it was a year ago. But we're still in a transitionary period that's merely starting.