You're injecting some of your own subjective opinion here. Law from his own mouth told us that he wasn't capable of taking Doffy down but put his trust in the man who could, he wanted Doffy destroyed but admittedly couldn't achieve it on his own. And I think you're missing my core point here, I'm not saying that it was bad writing for Luffy to defeat Doffy, in fact one can argue that many of the villains he defeats should have been taken down by someone else who was far more directly affected by said villain, but this in and of itself proves why Luffy will defeat Akainu. Luffy is the physical manifestation of freedom and hope, he vanquishes darkness with light only HE possesses as the Sun God, he is the hero who doesn't desire to be one. Looking at Akainu, as you stated in your post, he represents ultimate restriction, an absolute sense of Justice that has no room for freedom or independence, it promises the people that they will for eternity be held and controlled by the Nobles, who are the true darkness that plagues the current world. It is only befitting thematically that Luffy destroys Akainu who represents something polar opposite to Luffy. So this idea that you speak of is not something Oda will apply to Luffy since HE is the hero of this story and no one else.
It's been repeatedly implied that, with the exception of Garp, none of the Admirals are as strong as the Emperors. People might try to argue that this isn't the case, but it was made very clear with Shanks scaring off Ryokugyu that the audience is not supposed to see the Emperors and the Admirals as being on the same level. I really don't see Akainu being one of Luffy's opponents, despite what headcanons people may have about his strength.