I've never seen this discussed–which doesn't mean it hasn't been–so I will discuss it myself. Let's begin with a preliminary observation:
i) To my knowledge, the human body typifies planned obsolescence. Almost as though the body has an auto-destruct that triggers after it runs a fixed number of cycles. Each body has a natural lifespan. And that varies from one body to another. Jeanne Calment didn't live to be 122 because she was a health nut. Rather, it seems like the rate at which she aged was slower than for most folks.
I think human bodies are programmed to shut down at a certain age, although the program varies from one body to the next. Of course, it's not quite like a switch with a timer. Behavioral and environmental factors affect longevity.
But there's a sense in which the body is programmed to self-terminate. That's internal to the body. Each body's inbuilt lifespan.
ii) Now, one kind of life-extension is counterfactual. There are external factors, like pathogens, that can prevent the body from completing its natural lifespan. Take an illness that will shorten the life of the patient, absent medical intervention.
In that case, medical intervention counteracts the attack from outside forces. It restores the body to normal functioning, as if the patient was never sick in the first place. Resetting the status quo ante, so that a body will continue enjoy the longevity it was internal programmed to have. That kind of intervention prevents premature death. Keeps a body from dying "before its time". Let's call that natural life extension.
iii) Another kind of life-extension prolongs the body's longevity beyond its natural lifespan. Like resetting the timer on the auto-destruct. It impedes the aging process. This can involve medication, oxygen, surgery, transplants, implants, &c. Let's call that artificial life extension.
The difference between between the two is that natural life extension blocks something that interferes with natural longevity while artificial life extension interferes with natural mortality.
A potential problem with artificial life-extension is that a body continues to age. Like pushing the car's temperature gauge into the red zone. Organs continue to wear out. Organs and systems are now aging beyond the body's natural lifespan. That makes a person more susceptible to dementia, macular degeneration, &c. Even if a cure is found for Alzheimer's, if the brain continues to age, a person will still become senile due to brain atrophy. The brain shrinks with age. Because the brain has so much redundancy, you don't automatically become senile. But below a certain threshold, the deficient begins to manifest itself.
iv) Prolonging life in this sense can generate self-imposed dilemmas. For instance, Hans Küng is flirting with euthanasia because he's going deaf and blind, suffers from osteoarthritis, and Parkinson's disease. Yet he takes 10 pills a day to stay alive.
If he finds aging unbearable, why doesn't he just discontinue his medications? Why take pills that artificially prolong life while contemplating euthanasia to end it?
v) Apropos (iv), to refrain from artificially prolonging your life beyond the body's natural lifespan is not the same thing as suicide.
BTW, I'm not suggesting there's anything wrong with medical devices that preserve life. Although I oppose transhumanism, biotechnology can be a wonderful tool in medical treatment, both by saving life and enhancing the quality of life.
In addition, I'm not discussion public policy or bioethics. I'm just making the point that individuals need to consider the tradeoffs and consequences of their personal choices when it comes to prolonging life beyond the body's inbuilt longevity. Both Christianity and atheism have certain tensions in this regard.
In atheism, human life has no intrinsic value. Since, from their perspective, there is no afterlife, some atheists cling to life. Hang on to the last possible moment. Others commit suicide when their best years are behind them and they have nothing much to look forward to.
By contrast, Christians think life has intrinsic value. Yet that's balanced by their belief that this life is not all there is. Indeed, the afterlife will be better.