Assets mean nothing without a context.

What good is job AF without the actual jobs? What good is an end-boss without the actual environment and quest the fight takes place in?

Not everything is limited by context, though. They have considered it to be easier to release everything at once as far as some larger-scale improvements are concerned, like the animation and effect-related improvements. I wouldn't be surprised to see them do the same with the zone-related changes in the distant future as well.