To expand on what phil is saying: It is by itself logically consistent for someone to decide that there's enough fun left in their life and enough stuff that they need to do that they don't want to die now even if they don't necessarily mind dying in the indefinite future.

Humans don't live their lives as an exercise in exploring the rational consequences of consciously held principles. And actually, a lot of people don't go in for cancer treatment, because cancer treatment is fucking miserable and doesn't always extend your lifespan as much as you'd hope.