However, if the war ends on Onigashima, the people of Wano wouldn't know jackshit about Kaido's and Orochi's downfall. You can't tell me that this saga ends with the SHs going back to the Flower Capital and telling the people there "Hey guys, you haven't heard of it yet, but we just defeated Kaido and Orochi woop woop. You just have to believe us!"
Im sure the alliance getting first the asswhopping in Onigashima and retract first to Wano where the war starts around the whole country.
But the though of that the retainer getting killed is really dark, but it would give a spice into the story after all. Wano war it said to beeing even better then MF so I would really see some twist around the war arc.