Because Zoro would want to fight Mihawk and Mihawk would want to fight Zoro. You are ignoring narrative implications and writing decisions Oda is making.
You are right Cross Guild is irrelevant, SAVE for the fact that it is an avenue to involve Mihawk (and whatever other pirate group they recruit) in the fight for One Piece. Why do you think the entirety of 1082 exists, especially this page?
Do you think Oda just writes dialogue like this with no payoff?
Once again, this entirely supports Mihawk being invovled in the fight for One Piece, and thus his endgame with Zoro. I have still not heard one justifiable reason Mihawk needs to be "saved" for the WG or after.
Hell, if the WG arent the final villains (unlikely, but lets play devils advocate), nothing I said above changes, as Mihawk could then just be Zoro's final fight before One Piece anyway.