We already know Mihawk is tired of life, does that mean his will is now weaker than it used to be?
Even if this was the case, fighting Zoro, Mihawk will be the strongest he's ever been, due to Zoro reigniting that spirit in Mihawk by giving him the duel he wants, that only Zoro can give because he'll have surpassed Shanks as a swordsman.
So even if Mihawk was weaker than Shanks in terms of haki due to being tired of life, you can say the moment he's fighting Zoro he'll actually be > Shanks before he loses/dies.
He's already a stronger swordsman either way, but when Zoro beats Mihawk, he'll have surpassed both Shanks & Mihawk respectively. Zoro as a conquerors swordsman because guess what, Shanks is a conquerors swordsman just like Oden and Roger, who failed to ever beat Mihawk, whereas Zoro is a CoC user swordsman that will beat Mihawk, in terms of narrative the portrayal will be clear.
Trying to use the argument of "12 years ago shanks" doesn't work, or at least it won't work on me or anyone else who knows how much Oda values the narrative to get things across. Because for example, Oda said the reason the timeskip was only 2 years is because Luffy would have matured too much as an individual for the rest of the story to suit the type of story Oda is writing (Shounen).
This is why you have a 19 year old running around beating Yonko. Conversely the duel Mihawk and Shanks had doesn't need to be downplayed by saying they were somehow much worse, the fact of the matter is they both fought and no clear victor was decided, but we know that Mihawk has superior capabilities as a swordsman.
This is why I think when Mihawk is talking about settling things between them, it's actually about overall strength as a combatant, it's not actually solely to do with swordsmanship, because they both already know that Mihawk is superior in terms of swordsmanship. Shanks would have made up for that with his style of fighting with conquerors haki, and that also leads into his ability to cancel out observation haki and so on.
It's not just because Shanks lost his arm, catch on, Mihawk would be a better swordsman even with one arm, as in being more skilled. But the mitigating factor of Shanks losing a one arm is narratively used by Oda so that, because they both know Shanks is inferior in swordsmanship and now he's gone and lost an arm, even if Shanks could still use haki to keep up, in Mihawk's eyes any potential bout with Shanks is spoiled in that Shanks will never be at his very best blow for blow.
You will then bring up Zoro having one functional eye and Fuji and so on, but even this can be worked around with observation haki, I doubt Mihawk has the same ability as shanks to cancel it out. Shit if we find out Mihawk never had CoC and just became the strongest that way then that will obviously be the case.
Suffice to say being limbless is according to the WSS and oda's narrative reasoning a greater mitigating factor than visual perception.