I agree overall with you, but you are exaggeranting. Sabo has something to do with Ace: he is also Ace brother. And he is also the user of mera mera. Its possible a rematch of mera mera and magu magu
Relative to Luffy? Incomparable. Sabo is the physical example of a living will, thats the reason he inherited Ace's fruit.
Sure, they can "fight", same way 15 people fought Kaido for all of their story merits. It still doesnt supercede Luffy's merit and the catharsis the readers deserve from Luffy fighting him. He's the one that witnessed Ace being murdered and he's the one who was scarred by Akainu.
Sabo's backstory and history doesnt involve Akainu, no. He was scarred by Tenryuubito, he took part in DR to face another powerful Tenryuubito (which again, Luffy's story merits supercede Sabo, so obviously he wasnt fighting Doflamingo).
Even recently, Oda had Sabo discover the Gorosei and Imu, and virtually had nothing to do with the Admirals during said conflict, nor Akainu who appeared in Mariejoa later on.
Sure, I "exaggerated", but the logic that Sabo has any merit fighting and defeating Akainu in this story, especially over Luffy, is ridiculous lol.