We know that every Vice Admiral has some form of Haki.
Would it be safe to say that each Admiral has mastered at least one form, and that each has all three kinds?
Kaido said Haki reigns supreme, and who is the supreme force of the Marines?
Luffy is trying to become a top tier like the Admirals: someone with a strong Devil Fruit, who has mastered both their Fruit and their Haki usage for their fighting style.