Я не знаю

This is especially clear when looking at the similarities between Law, Robin, and even Nami Vivi and Sanji’s character arcs and compare them to Ace’s.

Law and Robin are both characters that accepted a bleak destiny for themselves at a young age and closed themselves off emotionally because of it. They, along with Nami, Vivi, and Sanji adopted a refusal to rely on others, all choosing to suffer alone and sacrifice themselves for others (Law by leaving his crew behind for dressrosa and trying to cut the alliance off with the strawhats when he expected to die, Robin trying to distance herself from the strawhats for their safety in Enies lobby, Nami joining the Arlong pirates to buy back her village and trying to keep the strawhats from getting involved for their safety, Vivi with baroque works and repeatedly having to be told to rely on others in Alabasta, and Sanji trying to distance himself in wci).

Ace also accepted he deserved death from an early age and constantly put his life on the line and refused to back down in effort to protect both Luffy and others. As a child he put himself in danger for Luffy and even Dadan during the fire, he put a wall of fire between his crew and Whitebeard the first time they met to ensure none of his crew were in danger and stopped them from even trying to help him in combat, he was horrified to see the Whitebeard pirates show up to marineford and wanted them to just let him die rather than risk trying to save him. He tried to push Luffy away and told him to leave the second he saw him at marineford. He refused to let Whitebeard be insulted and took the blow from Akainu for Luffy.

Both Luffy and the narrative itself refused to let all the other characters die (or live imprisoned) as sacrifices. All those characters were forced to accept help and love and then live with that knowledge.

Ace was not afforded that arc. The reason why? Because Luffy needed to learn a lesson. He needed to get serious. He needed motivation that comes from pain and loss. So Ace was fridged. His death, character, and narrative arc reduced to an emotionally charged device to further the plot and development for the protagonist. And now even as Ace continues to haunt the narrative, his legacy is reduced to a symbol of Luffy’s suffering and motivation to try harder.

Ace didn’t start as a plot device (at least to the fans and narrative, Oda himself always planned on killing him), but his death and the fact that it was executed with such a stark contrast to similarly themed character arcs suggests that had Ace been any less important to Luffy, he likely wouldn’t have died and instead would have lived on and grown within his own secondary narrative.