If haki was everything basically, then it would be too generic.
Thats probably what its gonna be like in the new world just like df powers were in the first half of the grandline

Another reason why i think Zoro Has Haki

You Have Shanks(confirmed Hakki User) before he lost his arm used to battle Mihawk evenly and shake the heavens. Theres no way mihawk was fighting evnly against someone as talented as shanks without using hakki himself.

Mihawk likes/ is interested in zoro (and luffy) I'm not sure how they trained but it definitely wasn't half-assed. I bet Mihawk beat everything he could into zoro so he could have a worthy opponent now that shanks is armless.