"T-There's nothing to suggest is stronger than Mihawk, he's only more skilled!"
Mihawk made the answer clear as day. They may have been equal in the past but they certainly aren't now. Mihawk isn't an idiot, if he thought Shanks could best him in a fight despite only having one arm now he would have accepted the challenge.
He got so bored trying to find worthy opponents he started hunting marines, but somehow we are supposed to believe he hasn't cared to fight Shanks solely because he is less "SKILLED" now? Do you troglodytes understand how stupid that sounds?