Mihawk doesn't want to fight Shanks because he lost an arm, but despite that his Swordsmanship is stated to be up there with Mihawks.
So either Shanks lost his dominant arm, but went on to be able to train his non dominant arm to be at a high level of Swordsmanship, which means if he still had his dominant arm, he'd be leagues above where he is at with his non dominant arm.
Or
Shanks use to fight with two Swords, and losing one arm diminished his Swordsmanship, as he can no longer fight with his strongest style.
No matter how you look at it, everything points to the fact that if Shanks didn't lose an arm, he'd be the WSS.
In Zoros case, losing an eye doesn't mean much in the grand scheme of things because CoO exists. It's not crippling like losing a limb. Just look at Fujitora.