This is exactly how Akainu will be defeated

What will happen:

Imu will order Akainu to do a great cleansing of the world. After he refuses (bc of course he does not want that) he will get Domi Reversied and forced to give the command to all Marines. The Marines will then have a civil war between those following Akainus orders (as Akainu would usually demand) and those disobeying and doing what they think is right (what Akainu would usually hate and see as treason). Akainu will internally beg everyone to disobey him, seeing the faults in his ideology. He will finally be defeated / liberated by a combined attack from Luffy (pirate) and Coby (disobedient Marine). This scene will mirror Roger and Garp beating DR Rocks and those are also the two characters Akainu wanted to kill at Marineford.

Argument 1: Philosophy

How can Akainu be defeated in a way that actually means something philosophically?

From his perspective he and his ideology are doing good in the world. He actually wants the world to be a better place and he thinks that the best way to create this better world is to be relentless and absolute in his fight against piracy. Marines who are less absolute in fighting "the bad guys" are the reason they were not able defeat this evil yet. If everyone just followed the rules and did as they were told, they would eventually achieve a save world.

Now, would he be proven wrong by Luffy just fighting and defeating him on his way to find the one piece / overthrow the government / get revenge for Ace? No, in his mind the lesson would be "I should have tried harder to kill him before". The same would be true for any other pirate.

Just imagine seeing Akainus backstory of him and his loved ones brutally suffering by the hands of pirates, him vowing to become a Marine and end piracy and then cutting to the present, seeing Luffy beat the absolute shit out of him. This does not feel right even if you also show, what bad things he did in his effort to end piracy.

Would he be proven wrong by being defeated by a Marine or Revolutionary that has a different ideology to his absolute justice? No. This would only mean that Dragon / Sabo / Coby / Kuzan or whoever fights him happened to be a stronger fighter than him. This would not change his mind about what the best way to lead the Marines is and this would not prove him wrong.

Akainu needs to see the errors in his ideology. He needs to see what can happen when people blindly follow his orders. And he needs to really see how evil the system is that he actively supported.

So the government he protected and propped up, ordering the most horrifying action possible, the near complete killing of almost all civilians, mirroring and far surpassing everything the worst pirates have done, will leave a crack in his world view. Then after being Domi Reversied and having to give the order to kill everyone to the Marines he will slowly see, action by action, why blindly following orders is bad. His most loyal followers, those with his own ideals of "Absolute Justice" will be the worst ones, while those who go against his ideology and do what is right will be the ones he desperately hopes to succeed. On top of that, some pirates including the strwahats will aid the good Marines and civillians, showing him, that not all pirates are bad.

Only then, when he has accepted that he was wrong, he will be defeated and liberated by Coby and Luffy. It will not be an act of anger or hatred, but an act of undeserved kindness.

Argument 2: Story

There are a lot of things that need to happen in the story, that would be achieved by this theory.

• Luffy needs to fight Akainu again
• Coby needs to at least confront Akainu again
• The Marines must have some meaningful role in the end game
• There must be a way to separate the good and bad Marines for the future after the final war

Here is why some of this might be difficult otherwise:

• Luffy needs to fight Akainu again -> By the end of Elbaf Luffy will be way to strong for the fight to be interesting in a dramatic way. Even if Akainu got ACoC, he aint doin shit to EOS Luffy, so instead of a hard fought fight, the tension will come from the emotional twist of Luffy liberating Akainu

• Coby needs to at least confront Akainu again -> Here we have the opposite problem, Coby can not beat Akainu in a full on fight, that would be too much growth since he was running away from fodder on Hachinosu. But what he can do is fire off one extremely powerful punch for a one hit finisher combined with Luffy

• The Marines must have some meaningful role in the end game -> There is not really a good opponent for the Marines, because it is very clear that the Marines are not a uniform faction. So them fighting each other is the best possible route that fits the moral greyness of the organisation. Also this gives weight to Garps life goal of protecting and fostering the good in the Marines.

• There must be a way to separate the good and bad Marines for the future after the final war -> After the WG is defeated, there still has to be a police force of some kind. If there is a civil war in the marines it will be obvious for everyone who actually fights for justice and can be a Marine in an ideal new system

Argument 3: Parallels

• Luffy and Coby vs DR Akainu will mirror Roger and Garp vs DR Rocks
• Those two are also the ones Akainu was going after at Marineford
• DR Akainu also mirrors Bellamy beeing controlled by Doflamingo in Dressrosa in many ways that I do not have the motivation anymore to type out