Luffy might have a confrontation with Akainu, but it will ultimately end up as Sabo vs. Akainu. You cannot really take Luffy's and Sabo's words as ultimate proof for your narrative, as the narrative building up goes beyond what both initially planned.
- Luffy said he needs to beat both Yonko and Admirals if he wants to become the PK. Problems: (1) He already beat Kaido, who is above the other Yonko and the Admirals, he also didn't face Big Mom, and it stands to question whether he will face Shanks. Just as Luffy never mentioned Akainu in particular as a fight he will have, but rather grouping together Admirals as a bastion standing between him and his dream of becoming PK.
- Now however, Luffy's role goes beyond becoming the PK. He is the reincarnation of Joyboy basically, whose purpose is to unite the world and face the leaders or the leader of the WG, which is Imu him- or herself, who is the antithesis to Luffy. Akainu as a (fleet) admiral is just a replaceable soldier (not the first and not the last (fleet) admiral), whose defeat won't change anything
- At the same time, while the Revos said that their enemies are the Celestial Dragons, they now come to realize that the scope is actually larger than they can encompass. Facing the King of the World for god knows how many centuries is not something the Revos aimed to doing; Sabo himself as the second in command was in utter shock to witness that there is a power standing behind the Celestials and even the Gorosei, who sits on the throne. Fighting Imu will be out of their scope.
- And while Akainu isn't a celestial, he is their greatest shield. The Admirals are said to be the WG's greatest military force and the admirals the direct dogs of said Celestials, with Akainu being the strongest of them and with the highest position. Sabo understanding the scope of events and understanding Luffy is the chosen one to bring peace and unity to the world, would be perfectly fine with taking over the fight with the Celestials strongest shield, while Luffy himself will aim for Imu.
- And while Sabo has no personal grudge against Akainu and said Ace made his own decisions, we know that Sabo inherited both Ace' power and will, with people mentioning how alike Sabo and Ace are, and with Sabo upon wasting Burgess, saying that it seems like Ace hasn't forgiven them and speaks through Sabo thus. So having inherited Ace' will and being Luffy's "new" brother who was believed to be death, it is perfectly reasonable for Sabo to yet again act like Ace, who stood between Luffy and Akainu, dying in the process of protecting Luffy, but this time, while Sabo will do the same (just like Ace standing between Luffy and Akainu), he will actually be able to compete with Akainu, while telling Luffy to proceed to the "real" enemy of the world he as Joyboy has to face.