Your biggest pro argument is the best counter argument. Its true that Big Mom didn't take any serious damage yet, but you shouldn't compare her to Kaido, who constantly tanking hits to his body, but also regenerates and endure trough his Zoan.
Big Moms power is based on a perfect shield, which cant be penetrated. She is an
Iron Baloon.
Once you get trough the outside shield, the damage will be fatal for her old human body. And there are no dragon scales or any zoan powers which will help her.
Thats how easy Big Mom can be damaged without her shield. Getting trough it == Her End
Katakuri and Cracker had the same flaw, as their fighting style was based on preventing damage, rather than tanking it with durability.
I bet that Kid will awaken his DF and show Big Mom how real pain feels.