Why doesn't BM have to die? Kaidou has to die because it's not like he would just stop wrecking havoc after his defeat. Why should BM just stop? Why should BB just stop?
Big Mom has set precedence for being a good guy, with Tama, with Elbaf, with another Caramel and the other kids. In Dressrosa Don Chinjao was an enemy until Luffy punched him to submission. I think Luffy punches Big Mom to a permanent OLin state. In this vein either by Elbaf or Wano, two emperors will have fallen like he has stated prior
and I believe BBs overconfidence will be the end of him. I don’t know anything about him so I cannot make any educated guesses. Also this is a shounen. Deaths are already rare. Kaido is a creature, and there is set precedence for creatures to die without having any narrative inconsistencies. Also largely due to Kaido being captured but not killed all the time