The story would be a lil bit different
Sanji likes: Cooking, fighting to protect people/stuff he cares about, being a ladiesman, wants to find a legendary sea to further improve himself as a cook, a kind man who likes helping the others, despite not liking to admit that and not being the best at transpiring that feeling with words (tsundere)
Luffy is: An idiot who acts inconsequent for the most part, can make friends easily, loves absolute freedom, wants to draw attention and make a name for himself around the whole world, is very good at fighting. Is mostly very honest with his words, most of what he says, he really means it.
Sanji is overall more relatable in real life than Luffy, i believe. For a shonen MC, i think Luffy fits it better tho
Take a look at Shokugeki no Sanji, that spin off is mostly about Sanji as a cook, we have no fighting stuff and it is still decent to read, we feel like we want more (the drawing is awesome and pretty different from One Piece too but you got my point).