a person who can be helped by a medical intervention will die no matter what, but if they get an organ transplant or a pacemaker or continuous dialysis or a cancer removed, they may live a little bit longer, as far as scientists observe, although sometimes with significant difficulties. immunosuppression and chemotherapy both deactivate certain organ systems.
no one can see the future. cancer treatments seem to help some people and be useless for others, and it might all come down to something like, "uh cancer stem cell #13491340 was not destroyed, and there was metastasis." the idea with tumor excision is usually to prevent there from being so many potentially-metastatic stems.