User blog comment:DancePowderer/What's the Deal with Cactus?/@comment-4899461-20130325153318

First off, I treat Sabo's death as a death currently. The following are reasons why I think his being alive could be good for the story:

I think that after all Luffy went through to save Ace, he deserves a brother. The sadness of the deaths of each brother becomes so much more sad when you take into account the death of the other, and the feelings of the other brothers. Sadder still is the fact that while Ace died knowing that people cared for him (aka fulfilling his dream), Sabo died long before he could travel around the world as a pirate, so he could never accomplish his dream. Ultimately, Sabo treated everyone (especially Luffy) fairly, and he deserved a lot better than what he got.

Now, to answer your question: I think if Luffy was to discover that Sabo was alive, it would become the single happiest moment in the entire series. With all the sadness surrounding the story of the three brothers, if such a happy moment came out of that, the happiness would be incomparable. I feel Luffy would be far happier if he found Sabo alive than the moment when he becomes King of the Pirates. I also feel like since the latter is such an eventuality that it would be hard to write it in a way that fills the reader with happiness. Without that, the only other moment that could possibly compare to the happiness of that would be the reunion of Brook and Laboon.

And with Sabo being considered dead for so long, I don't think it would completely ruin the story. The emotions created for the characters are just as strong whether Sabo is alive or not. No matter what we think, Luffy thinks Sabo is dead. Oda has done a fantastic job handling Sabo's death because at the moment when his death is most emotionally powerful (Ace's death), we didn't even know Sabo existed. It's the reader's obsession with knowing the "truth" before they make an emotional evaluation of Sabo's death that cheapens their reading experience. Luffy's perceived truth is what's important to the story, the reader's perceived truth is irrelevant.