Mihawk isn't strong as Saint Shanks
Mihawk bounty is inflated
Fighting Mihawk for years couldn't make Saint Shanks to be a yonko candidate even
Beating Loki made Saint Shanks be a Yonko
Mihawk stopped fighting Saint Shanks when he lost his arm. After that, he became a Shishibukai with a bounty of 3,590.bi frozen, without being a yonkou. If Mihawk had been defeated, like Loki was, then Saint Shanks could have become a yonkou. But Mihawk himself acknowledged that he stopped being a challenge without his good arm.
Now, Shamrock probably uses devil fruit as a supplement to be strong. Neither Saint Shanks, nor Mihawk, nor Oden, nor Roger, nor Ryuma, nor Zoro need devil fruit to be strong, which lowers expectations for Shamrock's sword skill (haki included).