User blog comment:Jonny16/What is the meaning for Oda having Sabo killed??/@comment-3119203-20111227172530

The point was to make Luffy realize that he was not yet strong enough to take on the world, and that he still had some growing up to do before he could become the king of pirates. If Sabo hadn't died, then Luffy would not have started on his path to strength, and him and Ace might not have been such great brothers. Also, Ace promised Luffy he wouldn't die after that, which was part of the inspiration that made Luffy go through Impel Down and Marineford. And if he hadn't done this, he wouldn't have gotten through the timeskip and would be floundering with the fishies in the New World right now.