This is not the final battle for Wano, whenever it lands on the Flower Capital or not Onigashima is definitively going to reach and land in the mainland of Wano and both Kaido and Big Mom have yet to reveal their true power. What we saw is only the tip of the iceberg and we still had a fourth or fith act to reach before the final fight against Kaido can take place.
And it's unlikely that Kaido will be defeated without his unstable alliance with Linlin ending with her either betraying him either to advance her own ambitions and plans, her going through another mind swipe though another BS from Oda's part or him turning on her first or Perospero doing something that will be successful at destroying this alliance he disaproves so much after all.
The fragility and unreliability of a partnership between former members of the Rocks Pirates has been brought up far too many times for it to not end in betrayal.