Seeing how Oda had both Big Mom and Perospero get defeated during the Wano arc, Oda could have brought Smoothie as well and have her get defeated alongside her mother and brother.
I can understand if he didn't want Daifuku and the others to join the war, but I think he could have at least brought Smoothie to the war.
Maybe instead of doing that dumb Kazenbo subplot, Oda could have had Yamato fight and defeat Smoothie.
Katakuri and Cracker were already defeated during the WCI arc, and Big Mom and Perospero got defeated during the Wano arc along with Kaido, King, Queen, and Jack.
With all these major characters on the Yonko side getting defeated, you would think Smoothie would get defeated as well along with them but I guess not.