Tough topic to address, but my brother has voiced something similar. My dad passed away at 87 this past May from colon cancer that spread to his lungs. He was quite the tough cookie & pretty much "healthy" up until the very end.
Dad used to take care of mom (take her to Dr appointments all the time (even if she didn't need to go), made her take her meds, etc..). Mom is 85 & has dementia. She recognizes everyone but is just very forgetful & is constantly losing stuff. She doesn't cook, feels she can live on coffee & cookies, or Ensure. She's quite frail but can still get around the house (albeit very slowly). She lives alone now but has a home health care aide that stays with her 5 days a week, 4 hours a day. Her helper is amazing & we adore her. I also help take care of mom. Stop by almost daily, do her shopping & sometimes even cook. I think I'm the only way that can "make" her eat, though very minimally.
After almost 60 years of marriage, mom misses dad terribly (as do we) & doesn't feel like going on some times. She barely eats, only if "I" make something & make her eat. She doesn't want to move into a home either. She barely wants to go outside of the house, so she just exists in the home, waling around day in / day out moving things around over & over again to the point that we are constantly looking for things for her.
My brother & I promised her (and dad) that we would keep her in her home as long as possible because she felt strongly, and quite frankly, they (government) just take all her money & all proceeds from the sale of her home to take care of her. We find that very frustrating because she loses everything.
I digress. My brother feels she is miserable & doesn't want to go on so why should she? It's a very personal, very difficult topic. Why can't we all pass peacefully at, let's say, 90? A good age, hopefully well-lived, and well-loved.