For Luffy to defeat Kaido the following needed to happen:
1) Kaido nerfs himself by lifting Onigashima
2) Kaido gets nerfed more by fighting multiple battles
3) Luffy getting saved multiple times
4) Luffy getting a power up (CoC Coating)
5) Luffy getting another power up (G5/awakening)
For Zoro to deafeat King the following needed to happen:
1) Zoro needed to realize that King can be damaged during his speed phase (no Enma and no CoC needed)
Hey, don't blame me, that's how your favorite GODA wrote it