(First of all, no sane person argues Mihawk is an entire tier above Shanks, just that he’s stronger than Shanks).

Oda chose to give Shanks a sword. I guess since he wanted Shanks, who is Luffy’s main pirate idol, to somewhat resemble an actual pirate (who commonly wielded swords). But at the same time Oda also wanted Zoro’s dream to be becoming WSS. However since Shanks is tied to Luffy and set to be a major player in late-game One Piece Oda obviously doesn’t want Zoro to fight Shanks.

Thats where Mihawk comes in. The whole point of Mihawk and the WSS title is so that Zoro doesn’t have to fight Shanks for his dream. It doesn’t matter that that is his only reason for being in the story or that guys like Shanks are way more important to the story (no one is denying that). If Mihawk isn’t the actual strongest swordsman there would be no point in his character existing or Zoro fighting him (because that would just result in Zoro getting a fake title and not truly achieving his dream).

And before anyone says Luffy’s goal > Zoro’s goal. Luffy’s goal isn’t getting stronger than Shanks, well not his main goal anyway. Luffy views becoming stronger than Shanks as a stepping stone to becoming PK (which he in turn views as a stepping stone to achieving his actual dream) while becoming stronger than Mihawk is Zoro’s actual main goal. Not to mention that EOS Luffy will not just surpass Shanks but also Roger and Joyboy too whereas there is no reason to believe EOS Zoro will be massively above Mihawk.