It's not about Shanks specifically, Mihawk benefits from all the other swordsmen, it's just the nature of the character, ofc you can always blame Oda if you are not happy. He doesn't need to show any feat, but for people like you who need everything spoon-fed to them, I guess you won't have to wait too long for Mihawk to show some god tier feats. Very excited to see more from Shanks too.
Marco knew what he was doing when he sent a strong swordsman (who happens to be way above your YC5 headcanon) after Mihawk, that was a very smart decision.