Katakuri and Luffy would likely have faced Big Mom, so an all out war.
Why would the captain be safe when her entire SC had been defeated? Especially since Big Mom wanted to annihilate the Straw Hats, Zoro would have asked Luffy to leave Katakuri for him and go fight Big Mom. And like what happened during Wano, Luffy would have defeated Big Mom somehow.
If Oda wanted the SHP to only fight the commanders, then Zoro vs Smoothie 100% and no need of adv.coc because Smoothie is just another Queen. Pre-Wano Zoro > all YC except King, remember only one needed Adv.CoC to be defeated, that's why Zoro faced only one commander and didn't go to WCI.