Let's face it Luffy is a hero. He is written to be one. What he's doing rn for Wano he'll do for the entire world in the future.
What narrative Akainu vs Luffy has apart from their personal beef?
Akainu is just like Kaido an oppressor but on a global scale as he's ruling the world's military which again watches over every part of the One piece world.
Luffy defeats Imu and dethrones the WG, what happens after that when nobody takes care of Akainu?
Will he just be voted out of office ? Will he retire ?
Akainu has to be beaten and what narrative point does it serve for Sabo to do that. I see no reason that makes more sense for Sabo to be his oponnent than Luffy.
Luffy never gave two shits about the Void century, but Dragon does.
Concerning the PL point, again, if Oda wants it something like Power levels will not stand in his way to make it reality.
I only see Akainu as a grand villain if he loosens himself from the WG for his own agenda. Until then, he just represents the outer face of the WG and is told to stfu by the Gorosei. In terms of Hierarchy, it is:
Im -> Gorosei -> Kong -> Akainu
What is Akainu's influence on a grand scale?
Imu: Ruled and enslaved the world for 800 years
Kaido: Enslaved Wano for 20 years, with the country playing an important role in the context of the history, with the WG being unable to lay hand on it for centuries
Blackbeard: Wants to destroy the world and/or put it into chaos
Akainu: Being an admiral just 2 years ago, with his main duties being serving as a shield for the Tenryuubito and executing orders of the higher ups. Now being a fleet admiral and in command of all the marines, yet still having to answer to the higher ups and having his arms tied when it comes to the most important decisions. Being (one of the) final villains would be anti climatic unless Akainu really goes berserk vs. the WG.