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First of all, Zoro’s role in the crew is being the muscle.
Whole reason Luffy seeked Zoro out was because Coby told him he was strong. Luffy didn’t seek him out to be vice-captain, since he had no idea that Zoro has that ability of leadership. Yes, Zoro may be the the vice-captain but he seldom acts like it, and the more day to day “vice-captain acts” are often conducted by Nami and Sanji.
Sanji’s role undoubtedly is that of a cook.
He is super strong, sure, but still his main purpose is keeping the crew well fed and providing healthy nutrition. Combat is his secondary purpose. It wouldn’t make sense if Sanji was as good in his SECONDARY purpose than Zoro, whose PRIMARY objective is being stronger and protecting the crew(That’s like arguing that Usopp should be as good a shipwright as Franky. He clearly doesn’t need to be, it’s not his job).
Secondly, most of the time we see Zoro in his spare time, he is training to become stronger, while Sanji is shown cooking or flirting with the ladies.
I am not saying that Sanji doesn’t train, he surely does, but not close to the amount that Zoro does. Not to mention that during the time-skip Zoro got trained by a Yonko Level character who specializes in the combat that Zoro does.
Sanji got trained by a Ivankov, who is strong too, but not nearly as much as Mihawk. Also, he didn’t even have the same method of fighting as Sanji like Zoro/Mihawk had.
Thirdly, Sanji adds so much to the crew other than pure strength. He is a world class cook, he does a really good job as a spy, he isn’t nearly as idiotic as both Luffy and Zoro, and he can fight really good in land, sea and air. In addition to this, he acts like a vice-captain in day to day activities and he’s loyal enough to die for his crewmates.
Zoro can’t do half of these things. If Sanji was as strong as Zoro or even stronger, then that means that Sanji is better than him at almost every thing. This simply can’t happen. All Straw Hats should offer nearly the same amount of usefulness in my opinion. It would be weird for Sanji to greatly excel Zoro in usefulness. So, the method that their usefulness is more or less equal despite Sanji’s many different talents compared to Zoro, is that Zoro should be better than Sanji in combat.
*Theory by HellUnleasher