What if he got advCoC through the fight with Kuzan? Haki blooms in the heat of battle and this was probably the furthest Akainu has been pushed in his life.
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If Oda thought this would be an issue he wouldn't have decided to include CoC on Sengoku's Vivre Card. Every Fleet Admiral will have conquerors haki.
That would be Super Disrespectful, cuz it means Yamato learned it without ever leaving her Home & barely Fought anyone, while Akainu could only learn it in his 50s, even though his Seniors (Garp & Sengoku had it), so He clearly knows about it. It also doesn't fit his Fighting Style at all
His Main Goal in a Fight is to Melt you or put a Hole in you, Haki isn't needed here
It's similar to Magellan's Fighting Style, any Opponent needs to avoid his Poison, whether you add Haki or not to it, won't change the Outcome
Sengoku is not just any Fleet Admiral, that's like saying throughout History, WG only accepts CoC Users as Fleet Admirals & We know They are Promoted from Admiral Position, so that means for WG to ensure FA has CoC, then They only put CoC Users as Admirals, which clearly isn't the case
Sengoku is like Roger of Marines (With Garp as his Whitebeard), He has "Reigning Justice", He is Buddha, literally a Religious Figure, He & Garp & Tsuru are considered Legends regardless of their Rank (Garp is a Vice Admiral for example), and when Roger was Introduced in Oden's Flashback, He didn't say "Bring me an Admiral", He said "Bring me Garp or Sengoku"
It's pretty clear that Sengoku & Garp are above just a Marine Rank, if you want to compare it to Pirates, Fleet Admiral is Equivalent of "Great Pirate", but just like not every Great Pirate is considered as Legendary as Roger & WB, Admirals are not considered as Legendary as Sengoku & Garp
Also if you check entire Manga before Start of Final Saga, you will find that Oda showed us Signature Attacks of every Strong Character whom Luffy met, except who? Shanks, Mihawk, Dragon, Whitebeard, Rayleigh, Sengoku & Garp
Literally the Group of Characters who are Hyped to have Roger Level Haki, Oda simply leaves Best for Last.
We've already seen Four Named & Very Powerful Attacks from Akainu, no need to expect a Major Upgrade, at Best We are gonna get two more Attacks & his Ultimate Move (Something Island Level), that's all
There is also the Fact that there is still another Promotion for FA which is becoming Commander In Chief of Mary Geoise like Kong, so it's not question of Rank, FA Position is same as Admirals, CoC isn't Necessary