Idk what Oda is doing with Mihawk and Shanks. If we take Mihawk's title at face value, Mihawk>Shanks.
however
Shanks has the best CoC feats in one piece bar none. Mihawk just doesn't. Mihawk is WSSM, he has better skill with a sword than shanks, yet Oda can't just come out and say Mihawk defeated shanks. He can't give Mihawk a higher bounty than shanks. He can't even have Mihawk split the sky with Whitebeard, despite them both being in marineford.
Mihawk is said to be Shanks's "rival", but Shanks has a much greater rivalry with blackbeard. In fact by his own admission, Teach is more dangerous.
Maybe it's like this. Shanks< Mihawk, however Shanks's CoC mastery> Mihawk's, with Mihawk solely being stronger due to having a black blade and 2 arms.
in any case, I don't think you can say shanks fans don't have a point, when Oda himself wanks shanks more than almost any other character.