The intent of the game is to raise questions about the way people live their lives, and if it is right to waste a life, even a short one. I hope that the concept behind the RPG does not offend anyone, as life-threatening diseases are extremely sensitive subjects, and even moreso to those who suffer from them, and the family members around them.
Why choose to simulate it when I could simply use it for a plot element? Well, I don't want it to just be a plot element. An ailment like this isn't just something somebody happens to have that bothers them, like acne. It's something that becomes a part of their life, and is a part that they're constantly struggling with. I don't think just "low vitality" is a good enough sign of Kayou's ailment... I want the player to have to keep on top of her health the same way the character has to, and at times, struggle like she has to.
Just saying "she's sick" seems like a pretty cheap way to handle it.