I don't know how you think this is going to go so smoothly. On top of Kaido and Big Mom, there is still King, Queen, and Jack (Who is recovering) to contend with. The odds are still stacked heavily against the protagonists.
The scabbards are out of commission, and who is going to be on the alliance side? Luffy, Marco, Law, Zoro, Sanji (possibly) Kid, and Killer. Drakes busy defending Chopper, everyone else will be busy with the Tobbi Roppo, Numbers, and countless fodder.
And then what happens when the rest of the Big Mom pirates show up, because you know they will eventually. For the alliance to win here and now, there will be some major assfuckery to have to take place.