Just going on a limb and saying that the comparison of Luffy saying he’s going to beat up Jack, versus Luffy saying he’s going to beat up Big Mom, are two completely different situations with different context. Big Mom and Luffy’s confrontation has massive amounts of build up due to their similarities but also due to the fact that Big Mom was the first emperor Luffy challenged the moment he came into the New World and they have an extensive rivalry with one-another. Not to mention that unlike Jack, she has multiple open plot threads that have been hinted to be expanded on due to the importance of her character and expanding on those wouldn’t really make sense with her losing to Kid and Law. Not saying that Big Mom ISN’T going to lose to Kid and Law, but I’m saying that her final opponent being Luffy makes complete sense and narratively has more backing than vice versa. Whatever happens, happens though 🤷🏿♂️