Thank you for phrasing this in terms of all-cause mortality. If only every academic study (and submission) used this as a metric, the world would be a vastly better place.
But in the context of something like prostate cancer, maybe all-cause mortality is appropriate for people who are already sick, because you are trying to figure out how the risk of death from the disease balances against the risk of death from the treatment, and where the latter may manifest in nonobvious ways, e.g. many people with prostate cancer actually die of heart attacks. Was the stress of cancer treatment a contributing factor? All-cause mortality statistics might help answer that question.