In their 60's people start complaining about the quality of life...especially here in the USA. I think other countries are different-especially countries where people work to live and not live to work. Those with better healthcare systems and better family policies at work seem to allow people to age more gracefully and with a better quality of life but here in America? It's hell to be old and unhealthy. Sometimes, it's just hell to be old.
Certainly one's own health is critical to "quality of life". My father has cancer (@83). If we were under the U.S. health care system, I doubt very much he would get the level of oncology specialist care that he is getting now and quickly/frequently: he was a restaurant cook before retiring @65 and no top-up private health care insurance by his employer. (One rarely gets that anyway working in the restaurant business).

It wouldn't be surprising how some people might find it easier not to follow healthier habits especially if they have significant preventable health problems: why bother and suffer @90-100 yrs.?

But I would disagree vehemently and my father is living proof that yes, you could suffer from a terminal disease but at least he has no other health problems to complicate the cancer. (No heart, respiratory problems...because his diet has been healthy all along.)

Most definitely a person does NOT want fall into habits that lead into multiple major health problems simultaneously in very old age. Then suffering is greatly compounded.