See, I think it would be interesting to get your take on it as a novelist.
If your main character's spouse/partner had a terminal illness, you'd have given it to him for a reason, right -- to raise the dramatic stakes, to deepen the level of pathos, to show your gruff main character's softer side, to wring the reader's emotions, to enhance the urgency of the plot etc. Eg Will X and Y manage to complete their mission/solve the mystery/achieve the goal before Y succumbs to his illness? Or how will X catch the killer while torn between being by Y's side and her professional duty?
But Y's illness still has to serve the plot, right? So, if this is, say, a police procedural, the mystery still has to get solved, so Y's illness can't actually prevent that from happening the way it would in real life, unless it serves the plot.
So, if it heightens dramatic tension for X to get a call from the hospital saying Y has collapsed and is calling for her while she is actually closing in on the killer, that could work well to raise the stakes and show X torn between love and her obsession with her job. But Y's illness is never going to mean that she hands the case she's been obsessed with for years to a colleague, takes compassionate leave, and we spend the last ten chapters in a hospital with no crime plot resolution, apart from X getting a text saying 'We caught him.'
I think SW rejigged the diagnostic timeline to strengthen the 'hook' of TSP, but at the time she almost certainly thought of TSP as a one-off.
Its success and the public appetite to know what happened next and whether TW had survived meant that she then had to contend with writing two sequels where she now has to figure out how to circumvent the supposedly terminal illness with a short prognosis she gave one of her main characters.
She can't kill him because this is a (technically) memoir, and he's still alive and comparatively well, and anyway, their devotedness is part of their USP, and she can't give him a miracle cure without the original diagnosis looking like a misdiagnosis and retrospectively lowering the stakes that made TSP such a success.
So she has to 'continue' his illness and put him closer to death, but that can't get in the way of the other USPs of TSP -- (1) musings about salt air and the natural world and healing, and (2) walking.
So he's dying, but only when it doesn't get in the way of (or when it actually promotes), the other stuff the reader expects from the Raynor Winn brand.
Hence in both sequels TW is ill enough to need to be rescued again by another healing LD walk, and both times is ill enough for the helpful Greek chorus of Dave and Julie to express severe reservations about his ability to complete either, but not so ill that he needs to be helicoptered out of mountainous territory to the nearest hospital and risks making SW look irresponsible or cruel. .