Everyone has their goals and needs, doesn't mean they will all achieve them. Saturn is likely to be defeated from a narrative perspective, even if it has to be 3rd party intervention.
The flashback with Saint Shanks dad and the fact Saint Shanks was about to depart Elbaf, could hint at Saint Shanks heading over to help kill Saturn himself. Or maybe a different crew will help Luffy, Dragon still didn't make a move in forever, now is the time to do something and for Sabo to maybe kill Saturn.