The thing is that Mihawk is not gonna be the final opponent in OP. For Zoro, yes but you got guys like Roger, Whitebeard, Xebec, etc. who is likely stronger than Mihawk.
I am off the opinion that Zoro will fight Mihawk after Luffy becomes pk. Reason being is that he told Kuma his ambition is useless if he can't make Luffy pk.
There is also the fact that final war will happen after op is found. So if Zoro participates in the final war then goes to fight Mihawk after the war, imo Mihawk will be stonger than Roger, Whitebeard, Xebec etc. WB said the final war will be the biggest the worl has ever seen iirc
If he fights Mihawk before Luffy becomes pk then I can see Mihawk being weaker than Roger etc