GreyLord
Legend
That was introduced in the prequels. It was never about that in the original trilogy. It was all about Vader's move back to the light side of the force and saving Luke.
Once again, depends on what you consider the storyline or not.
If you consider the creator's intent, the prequels would be part of the entire story.
Of course, if you discount the prequels and ONLY look at the Original Trilogy, it still is more than that.
Luke wasn't supposed to simply change his father to the Light side...that was Luke's belief that he could change his father, but that wasn't the intent of the original trilogy.
In the original trilogy it was a fairy tale of the good guys (rebellion) vs. the bad guys (evil empire).
In this, the entire intent was to defeat the empire, and like all nice fairy tales, that means you beat the big bad guy and everything else falls after you defeat him. Thus, Luke wasn't supposed to just defeat his father, but defeat the emperor too. In fact, defeating the emperor and the empire was the entire focus of that trilogy if you don't include the prequels.
If you just only look at the first movie, it was at least destroying the big bad evil super weapon that Darth Vader was on...not even turning Vader to the Light. It was still defeating the Empire (at least in the destruction of the Death Star) after saving the princess from it.
Vader turning but not defeating the emperor or the empire (thus having the rebellion defeated or destroyed) kind of leaves that as a hollow unfulfilling trilogy.