There is a huge difference between trying to fulfill your goal as the protagonist of a Shounen manga and trying to accomplish or obtain something without being the protagonist of a Shounen manga. It may be news to novice readers like fans of the admirals, but in Shounen manga, the protagonist is privileged. They receive plot armor in their favor, some are known for their immense luck, their ability to conquer and influence people, make allies, and even come back from death, become a god, or change their Devil Fruit abilities. This level of plot favoritism is bestowed upon the protagonist. If Akainu were the protagonist of this series, he would receive these same privileges and perhaps even more, as he is weaker than Kaidou, Whitebeard, and others.
It is for reasons like these that I acknowledge the Yonko. Even without possessing the same protagonist privileges, they have come closest to obtaining the One Piece, with Kaidou and Whitebeard being two characters who are stronger than Akainu. However, since they are not protagonists, they have not been able to obtain the One Piece.