I'll be totally honnest but those who hoped that Kid would have his 1v1 fight against Big Mom, that he would beat or kill her or that he was going to be Luffy's main rival and equal were blinding themselves about what was visible from the beggining.
It has been too evident since the end of Dressrosa arc that Luffy was already above the other Supernovas and would have the golden treatment compared to the others, with him defeating Doflamingo and getting his own fleet while Law was relegated to second knife rank. Whole Cake Island just confirmed it with Bege being so below Luffy and his best action being of outsmarting or shooting a bullet at Oven while Luffy attacked Big Mom and fought and won against two Sweet Commanders and was acclaimed as the Fith Emperor after that. There were no chances for the other Supernovas to catch up after that, and Wano just dug the nails in the coffin.
And that's not counting the fact that Blackbeard has been presented countless times as Luffy's main rival and competitor for the One Piece and the Pirate King title.
Besides the ties between Big Mom and Elbaf foreshadowing her presence on the island in the future and her unresolved issues with the giants have been teased many times.
And to finish Oda just doesn't kill anyone in battles or wars anymore and no major antagonist of One Piece has ever died in the manga, even if defeated properly, which didn't happen, Big Mom was never going to be killed by Kid or Luffy or anyone during this arc or after.