Each case is different so I can comment on the above, but...
I had cancer when I was 19 and would never go through chemo again. not putting up a fight is sorta committing suicide right? Also I wouldn't want to go through life with a severely dehabilitating injury or mental degradation.
I want my kids, family and friends to remember me fondly, not remember changing my diapers or forgetting their faces.
My father is showing some VERY early signs of "old age" forgetting things, deafness, frailness, et cetera. If he is following the path of my grandmother I don't expect him to come back from a hunting trip at some point. I don't begrudge him at all.