If a person is left on life support for as long as possible, regardless of ethics, morals, etc.
Why can’t the body heal enough to come off life support?
After my father’s stroke the surgeon carefully explained to us that one entire hemisphere of his brain was dead. There was not a question of “recovery.” The tissue was dead. Life support would keep the blood oxygenated and flowing, but it wouldn’t bring his brain back.
Similarly, a “heart attack” also involves the death of heart tissue. Life support can take over the function of the heart, but the heart won’t repair itself.
I’ll leave it to others to talk about other types of life support.
A body requires a living, functioning brain (or specific portions thereof) to regulate some aspects of its physiology, like body temperature and blood pressure. Without that control by the brain, eventually the physiology of the body gets farther and farther away from normal/healthy, and more and more individual cells die, and/or runaway infection sets in.
Conflict often arises between medical professionals who recognize the implications of brain death, and relatives of a brain-dead patient who do not. For a recent example, see Jahi McMath.
The Terri Schiavo case was slightly different, in that while her cognitive function was permanently gone, she apparently had adequate brain stem function to enable long-term physiological stability, including unassisted breathing; all that was necessary to keep her alive was a feeding tube, since she could not eat food. She survived for fifteen years like this, succumbing only after her feeding tube was removed.
For patients who are not brain dead or suffering from a permanent loss of cognitive function (e.g. coma patients), various degrees of life support can indeed provide them with the time they need to return to normal. The longest coma recorded was 42 years, though the Wikipedia page does not mention what degree of life support was required in that particular case.
Some health conditions don’t involve gradual healing, but gradual deterioration instead. If you have a cancer taking over more and more of the body, time is the enemy, not a friend. I think generally “terminal illness” indicates this.
Also, life support doesn’t necessarily support life very well. If for example the kidneys have completely failed, I don’t think dialysis works perfectly and provides complete life support in that sense. For sure, if the liver completely fails, there’s no life support tool that works around that problem. Body organs interact, and some of them die when others malfunction.
One of the earliest “right-to-die” fights was Karen Ann Quinlan. When her parents finally got the court’s permission to discontinue life support, she lived on in a vegetative state for almost a decade.