If Oda wanted the chaos of Big Mom's army in there, he would have started with it in there, (rather than making them so innefectual they can't climb a waterfall) and then at some point when Big Mom switched sides it would have been a truly chaotic free for all and we'd be getting nothing but crazy outcomes and team ups.
But the time to set that up was 20, 30 chapters ago.
Oda's already pressed to handle what he has, and he skipped the long hyped sulong mink fight entirely and had the Jack stuff totally off camera.
It was a cool idea and would have been an insane twist but we're not getting a repeat of Thriller Bark here. Plus, that ultimately would have devalued the heroes if you said at the end o the day "Well they only won because Big mom's crew did most of the work." Look how unsatisfying that is with one fight, and imagine that applied to the entire army.
And Oda's certainly not having the crew take out a yonkou crew by the skin of their teeth, and then turning around beating another one immediately while they're half dead.
It'd be n intense "oh shit how are they getting out of this?" moment to be sure, and for that reason it might still happen after Kaidou is beaten, but then you need something like the grand fleet or Shanks or the marine admirals arriving to resolve it. And those would all be bad ways to handle it.
Peros is there to give some lip service to the Carrot plot that won't have time to wrap up later, and Big Mo because Oda changed his mind about beating her on Cake Land at the last minute (because that would have taken another year at the time to properly wrap those threads.)
@Deicide:
If Oda wanted to rush this, he could, Act 3 would have looked very different, however. He is taking his time to set things up.
At one point do you stop moving the bars on your insistence of the five act structure for the action? If they were going to having a dramatic downturn moment, that time was when Luffy was taken out and they were briefly losing hope.
But that was undone the very next chapter.
Third act is already longer than the first two combined , and unless it stops specifically for the flashback or something, there's no longer a really good moment for it that won't kill all the momentum. I guess the island crashing and the fights spreading could be a thing? But Luffy is coming back, not just waiting for them, so…
Two of the fights are already done, and the rest are all setup and underway, outside of Zoro getting back up somehow to fight King.