Shanks' legendary status comes from his unparalleled Haki, not a Devil Fruit. His sheer power, evident in feats like stopping wars and intimidating admirals, would be diminished by one.
As a Fish-Man, Jinbe's incredible strength and unique combat style are rooted in the sea. A Devil Fruit would strip him of his underwater prowess, negating his natural advantages.
The Pirate King, Gol D. Roger, achieved his legendary status purely through Haki and swordsmanship. Adding a Devil Fruit would undermine the raw, unadulterated strength that defined his journey.
Zoro's unwavering dream is to be the world's strongest swordsman, achieved through relentless training. A Devil Fruit would detract from his pure dedication to the blade and Haki mastery.
As the World's Strongest Swordsman, Mihawk's prowess comes from his unmatched skill and Haki. A Devil Fruit would only complicate his elegant, powerful fighting style with Yoru.