I don't have the source I do remember very clearly Yoshi-P said that Heavensward will be like ARR with a full complete storyline straight at launch. With an endgame more story added in patches like ARR. So no need to worry.

They also have gone out of their way many times to emphize how Heavensward's launch is being treated like a full fledged Final Fantasy release. Which means bosses/final dungeon and all that jazz right at launch.