Okay listen, I know I already wrote a fic about this, but I just love McCoy and Xenopolycythemia, and apparently 13k words weren’t enough to get my point across.
I don’t think McCoy just gets to accidentally find out he has xenopolycythemia. The real equivalent, Polycythemia Vera, is often diagnosed via exclusion, meaning you see signs that could suggest a lot of different diseases, and you slowly rule out the causes that you can test for until you are left with one option.
So McCoy doesn’t get to just suddenly find out he’s dying. He has to intentionally march towards it, each step removing more treatable diagnoses that he doesn’t have. It’s a conscious choice he has to make, and it’s slow. His chances get worse the more he continues, but he keeps going anyways, and when the last test result comes back, he’s ruled out every other disease. He’s left with nothing but Xenopolycythemia. And I think it’s more painful for him that way. He has to be an active participant in his death sentence.