One of the big problems with Sabo returning alive, is just that he is a lot less complicated and flawed as a person than ace was.
That worked fine when his role in the story was as Luffy's older, more cool-headed, nicer brother who died early, forming a good, memorable trio... But when you have him return and take away Ace out of that equation, you have this unintentional meta that here comes luffys better, far more boring brother to replace the dead ace.
We remember luffy and and ace's interactions a lot, with every moment they had at marineford having become iconic in some way.
Meanwhile, yes, we remember Luffy and Sabo's reunion well, but honestly, does anyone remember any other dynamic or great character moment the two of them had together as adults?
Sabo isnt a blank slate character at all... But when it comes to his most important relationship, his brother luffy, he just doesnt leave any lasting impression beyond a deep shared loyalty, and that is nice and all, but it doesnt lead to an actually interesting relationahip.
I would argue that there was a long stretch of time that Ace was as uncomplicated and boring as Sabo is now. He had his introduction during Alabasta, but didn't get fleshed out in any meaningful way until Impel Down/Marineford. In theory, Sabo could have some similar "iconic" moments with Luffy during the final arcs.
Thats not to say I think the manga justifies his need to exist. I think the story would be stronger without him. But there is enough difference that they're not 100% derivative, and for a long time Ace was just as popular for no real reason either.
Part of me wonders if Oda made Sabo so that Luffy would have more of a reason to care about the Revolutionaries, because it would be out of character for him to care about what Dragon was doing.
Luffy's got a 'neutral' sort of selfishness, where he will move mountains for you if you're his friend, but that doesn't necessarily mean he cares about some king being corrupt somewhere. His primary motivations so far are usually to do with his personal relationships.
So Ace dying and being replaced with Sabo is like... 'the times world-building direction it is a-changing' in that Oda starts expanding from 'King of Pirates' to 'the rest of One Piece's world'.
Ace's lineage and Whitebeard and all that is still very confined to 'piracy', whereas the Revolutionary Army encapsulates things far outside of that.
(What about Robin tho'? -> Robin finding out the True History doesn't necessarily mean Luffy would care about the world beyond 'Robin achieved her dream', I think. Luffy will declare war on the World Government for Robin, but he didn't do that because he fundamentally disagreed with what the World Govt stood for, he did it because Robin was his friend.)


















