You are right. I actually get that and you make it pretty clear that this may be what is going on. I guess the point I'm trying to make is this:
I entirely believe that Shanks and Mihawk are Luffy and Zoro's final fights. Emotionally, thematically, to me this is what makes sense.
With that being said, and with the ambiguity of how their matches seem to end, I would expect Luffy and Zoro to be pretty equal by the end. Thats fundamentally all I'm saying. When you look at a matchup with Blackbeard, he already has someone like Shiryu in his crew, which sort of suggests the idea that maybe Blackbeard isn't the last fight.
Because Shanks and Mihawk really only have associations with each other, I expect Mihawk to probably ally with Shanks in the coming final arcs. After all, he's going to be involved and he probably needs to be on some form of war front right? I would expect it to be with Shanks.
This of course, is if you are one of the people that believes Shanks isn't exactly who we think he is.