Someone think Oda made a mistake. He should of never killed Ace. Ace had lot of potential. He could of done much more things with Ace like make him the next WB and lead the Marco pirates. While
that happens the story would focus on Luffy and the shs and towards the end of the story Luffy becomes the PK while Ace continues with WB's legacy of big family.
And its wrong to say that Luffy needed Ace's death to become stronger. The reason he trained in the TS was because him and his crew got their ass handed in SA. Luffy actually wanted to quit being a pirate because of Ace's death. That wasn't a motivation for him to get stronger. What motivated him was the fact that his crew lost in SA and that he needed to become stronger so that the same thing doesn't happen.
But the author is always right. It might suck and be lame but they know what they're doing. They know where the story is going. I loved Ace but I think it was necessary for Luffy to understand he was weak compared to the others (by others I mean the new world pirates). Anyway we can't "undo" what was done we have to live with it. And as for ace's devil fruit it's not necessarily lost imo someone might get it in the future, that's something we can never know until it happens/doesn't happen.