Welcome
Mind to tell why change side? Lol
Because Oda has put himself in an ineludible situation.
If this is it for Kaido and Big Mom and the magma kills them/severely hinders them for the rest of the story, they were literally defeated by 3 upstarts in the span of a night and without DIRECTLY showing and stating Awakening.
They can't come back and be a menace because there are too many signals that this is over and Oda would have the BALLS of the millenium to justify Luffy, Law and Kid still being able to put up a fight against them. Specially because Oda has gone out of his way to show how IMPORTANT it is for the people of Wano to have their matsuri (Fire Festival) not only uninterrupted, but also lead to the unexpected news of the victory.
If it's
just Kaido that comes back, Oda can't write interesting scenarios for him that don't involve bullshit like "wow Adult Momo is literally identical like Oden and he's also as strong as he was because of his blood! He can give Kaido the final attack!" or "X character suddenly appears and kills Kaido and robs Luffy of some part of his win". And also he makes Big Mom look even WORSE in comparison.
Either way I don't see him turning this and making a good surprise. And as remarked by the last chapters of Kaido vs Gear 5, Luffy is still weak to cuts and very hot fire, which figures because he still has to challenge and defeat Shanks and Akainu AND his top two men are Sword and Hot Fire as well, lol.
Just a fucking disappointing resolution for the 2 most important villains of the whole story so far but it was to be expected since Oda decided that FUCKING LAW AND KIDD were going to face Big Mom in spite of having NO BUSINESS OR BEEF WITH HER WHATSOEVER.