White Magic limitations?

[Draxis_Fallspear]

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.

[Waliel]

When Y'shtola was sliced by Zenos, weren't the healers present doing more of an emergency first aid and she was taken to real doctors afterwards? I'd imagine lore-wise it would be longer process than just casting cure and sending the patient away. Like in combat they'd be able to keep you going long enough for you to not just fall over, but you'd need some longer treatment and rest after the fact.

...So it's just magic morphine and in reality we were heavily injured in the ARR opening cart, and now we're just seeing dreams and hallucinating while overdosing on cures.

[Zojha]

I'd imagine lore-wise it would be longer process than just casting cure and sending the patient away.

In the Astrologian quests, it's quite literally casting Aspected Benefic and sending the patient away.
"Heishia
What is this sorcery? My cuts... They knit themselves! Are you a healer, adventurer? If so, I have another request of you."
"Heishib
How did you...? It is as if I was never dealt a single wound! What manner of magicks do you wield, friend?"

Etc. You do that a lot in the 50+ quests and many of them are in their death throes to boot. Truly, many of them are sent back several malms through the snow right away. While conjury might possibly be a little less powerful, it's unlikely White Magic would be.

That said, the reason why we can't just heal everything away is simple: It cheapens the narrative.
Just as healing trivializes mechanics if you can just comfortably heal through the damage, it also cheapens the narrative effect of pain, death and disease, as it erases the consequences and thus impact and effect.
So you're given limitations to preserve those consequences and those are commonly arbitrary: You cannot dispel "that" debuff, you cannot rescue "that" person etc. Or as a compromise, you can "barely" save them, but they need to jump through X, Y or Z hoops afterwards. Why? Because whoever made that encounter/story didn't want the effect to just be completely erased. You can instead have your healer power fantasy on inconsequential random NPC #471 instead. It's that simple.

People generally underestimate how dangerous healing is on a meta-level.
To cope with it, the narrative adjusts healer power to the desired level to achieve the desired effects. Sometimes you can fix a dying person up with a single cast, other times they need months to recover and still struggle and other times you just outright cannot do anything.
The content instead adjusts boss power - hence easy content has lots of mechanics that can be healed through, hard content has instant kill and instant wipe mechanics galore.

Just crack it up to function over form.