The way I see it, the heart problem Sanche has was a physical defect moreso than an actual disease. As far as I know (which isn't much since I'm no doctor), heart disease isn't really a curable disease. You can unblock clogged arteries and control the problem with blood thinners to help the flow, but I don't think you can "cure" it (outside of changes to diet and exercise, I guess?).
That being said, I think that White Magic probably could cure these issues in the past. However, keep in mind what happened after Amdapor. The elementals were not pleased. I would assume that the elementals would believe in the cycle of life and death as this is the natural order of things. We probably aren't allowed to use magic to cure natural causes of death so that the elementals don't get pissed.
