I've wondered about this before.
In the old days if you were in such a condition, you'd simply die for lack of adequate care and medications. Even if people tried to keep you alive, it would be of little help.
Nowadays we have machines and medications which keep people ticking long past their expiration date. It is quite unnatural.
Nevertheless, the preciousness of a human rebirth is to be cherished and we should use our pain as a means to fostering our compassion and tolerance. That being said, such practices are ultimately only going to be successful for a small number of people. The average person, Buddhist or not, in need of 24/7 healthcare is probably not going to be able to practice as such.
In some situations cutting people off life support means they die from dehydration, rather than falling unconscious right away. That would be easy enough to go through, but after several long days dying from dehydration and organ failure will not prove pleasant, unless the patient is completely sedated.
I suppose if the doctors and staff assisting in the suicide are motivated purely by compassion and not desire for gain, reputation or pleasure, then the deed might not be that unwholesome. The Yogācārabhūmi-śāstra suggests that someone motivated by compassion who slays another person who is about to slay an arhat will actually gain merit. Not all acts are black and white. There is a lot of grey area between.
Assisted suicide is actually a similar case. Normally, taking the life of someone is unwholesome, but if motivated by compassion and both parties consent with full knowledge of the situation, it may not actually be an unwholesome act.