I don't think people change their mind on this kind of decision. For example, I could list in the document (written when healthy) that if I am unable to clearly communicate my wishes either verbally, by blinking, or whatever, maybe for more than X days, then I prefer to err on the side of dying.

A big advantage of having this written (and binding!) is that it takes the burden off of family to make the decision. Many people cannot make this kind of decision for others when they are incapacitated.