I for one always believe that to reach the very top, you will need everything in your favor.
More hardwork means a higher advantages. See Coby situation.
More talent means a higher advantages. See Yamato situation.
More destiny means a higher advantages. See CoC users.
So if you want to reach the top, you need all the things that benefit you. Someone with hardwork, talent, and destiny (for example Zoro) will possibly have a higher ceiling than one with only one of it (for example Sanji only have talent, but no hard work & destiny).
In this BM & Kidd Law fights, Oda wants to show us that hax DF can compensate one's lack of Haki.
Whereas Luffy with both hax DF & Haki can go further than them.