I agree.
But there's a thing. Time.
If Shanks gets his dedicated arc, then it's Zoro vs Beckmann. But then what? There's this "big war" being prepared by Oda, but I think it's REALLY weird a Zoro vs Mihawk duel happening in the middle of any kind of conflict. With Mihawk being back to flying solo, I find it fitting for him to appear in the same arc as Shanks, due to their friendship and all. the SSG apparently developed a power strong enough to replace the Shichibukais, which includes Mihawk. If they fail, then it kinda adds up to nothing, you know?
If the SSG is supposed to ACTUALLY be a threat, than Mihawk being solo should not be a possibility for him. Despite liking Posthawk, it only fits narratively that he isn't able to stay solo.
By going this route, and Mihawk coming into play during Shanks' arc, than it's a whole new dynamic for the arc.
But honestly, I don't think Oda would do this. :P