I get that but Bonney doesn't work as the catalyst for Sanji becoming a King. Him saving his princess wife by becoming a King works thematically and would be the far more likely route Oda takes if he want's to give Sanji CoC. Zoro becomes a King to protect his captain, Luffy becomes a King to save his brother, Ace becomes a King to save Luffy and Sanji becomes a King to save his princess, it just makes more sense.