Firstly, One Piece power scaling is not linear. This is not Dragon Ball. It’s perfectly acceptable for either character to come out as the victor or for it to end in a tie depending on the direction Oda wants to take things.
However here are my reason why Luffy should win this fight against Katakuri: Luffy losing this battle could have some major negative implications about the future of the story. After Whole Cake Island, Luffy will continue fighting Yonko commander-level enemies. If he comes across someone who is even stronger than Katakuri, people will also complain that “Luffy doesn’t deserve this win!!!1!!!1!” because by their arbitrary standards and misunderstandings of how combat works in One Piece, he “shouldn’t be strong enough to beat this guy because he couldn’t even win against Katakuri”. This same logic will loop and we’ll be caught in a dilemma of people both wanting Luffy to finally win another fight and yet simultaneously believing that it’s “illogical” (as if that even matters) for him to win in the first place.
Luffy is strong and has consistently defied logic throughout the series by defeating opponents people insisted were too far beyond his reach. Conflicts in One Piece aren’t about deciding the tougher brawler; they’re about deciding who has the greater ambition and willpower.
If Luffy isn’t ready now 900 chapters & 2 decades in, then when will he be?
*Theory by Atheistsomalipirate