How do you know Shanks has superior haki to Mihawk?
Have you seen Mihawk's haki being weaker than Shanks's?
Mihawk has the better haki feat of creating a black blade, something Shanks cant do but how then did you reach the conclusion Shanks has better haki?
remember Oden which was praised with other individual by Kaido (that include Shanks,White Beard,Roger,xebec) coudnt get a blacken blade. The dude had advcoc,monster haki, was a beast that left ptsd for over 20yeasr to kaido.
Oden's father knew very well his son, and he saw more potential in Zoro. Mihawk which is know for his blacken blade surely is a monster in term of haki. If unlocking advcoc aint enough to get a blacken blade, Zoro need to awaken a new power to get achive that.