Oda: "DF superiority doesn't tell you about the outcome of a fight" - Sabo vs Akainu is the only instance to prove that statement. Obviously, prime Sabo will be much stronger than Ace before his death. He already is known the emperor of flames, will probably awaken his fruit.
Also, Luffy has no grudge. He accepted that Ace died as a pirate and remembered his friends he wants to be strong for. With Shanks also saying that it's ok to cry, but at some point a man should look forward.
Sabo vs Akainu hence feels more logical narratively speaking. Sabo inherited Ace' power and will and is now Luffy's recurred Brother who will stand up for him and defend him. While Joyboy certainly has more important things to do than fighting the outer face of the WG, which is "worth less than zero" according to the Gorosei.
None of the excuses for Sabo will ever work when any sort of reasoning works a hundred times better for Luffy.
Sabo potentially having awakening doesn’t change much when the same applies for Akainu who has the superior fruit. That would just bring things right back to square one.
Luffy certainly has a grudge. Just hearing his brother’s killer’s name starts the large prominent scar on his chest aching. Even when his greatest pirate rival’s name didn’t bother him in the same context.
More importantly, Akainu himself has a grudge with Luffy. It’s the only mutual conflict in the series that’s this long lasting. When both Luffy & Sabo were in Dressrosa, Akainu couldn’t care less about Sabo while his focus was on Luffy and he even banned an Admiral from any Naval base until he brought Luffy and Law’s heads for him.
Luffy’s greatest failure, the main reason for the time skip was failing to save his brother due to his weakness. The obvious resolution to that is being strong enough to now do so and not have loved ones dying in front of him.
Having someone else do it for him defeats the point of the narrative.
On the opposite end, Luffy becoming Pirate King will be Akainu’s greatest failure as the Fleet Admiral. The men at the top of the Pirate and marine worlds, opposites in every shape and form… it’s a no brainer.
It’s storytelling 101. The antagonist that kills the person dearest in the world to the protagonist is going to get their comeuppance at his/her hands. It’s simply the maximum conflict and emotion that can be pulled from a story.
I find it odd in itself for someone to even want anything less.