The Admirals are so fucking useless to the manga that they can’t even properly work as obstacles for Luffy. A Luffy vs. Admiral fight to the death, going all the way until a winner is decided, where Luffy actually gains something from it and gets closer to his dream of becoming PK (Road), or whatever the hell he wants now? We’re never getting that from an Admiral like we did with Luffy vs. Kaido and Luffy vs. the Emperors.
Luffy gains absolutely
NOTHING from defeating Akainu, Kizaru, Greenbull, or Fujitora.


One thing is risking your neck, your crew, and losing a lot of members and allies in a war against Kaido and Big Mom, because if you win, you take over the territories they left behind, get their treasures, get their Road Poneglyphs, and take one step closer to becoming the PK and achieving your dream.
Another thing is going to war against Akainu or any Admiral, losing military power in the process, putting your crew at risk, and then walking away from the battlefield with absolutely
NOTHING to show for it. The fight contributes
NOTHING to Luffy’s dream. It doesn’t move him forward in
ANY way.
Beating an Emperor = Higher bounty + territory + status + treasures + Road Poneglyph + one step closer to achieving his dream
Beating an Admiral = Nothing + Losing military power in exchange for NOTHING.