What JOM describes is what I had to go through with both parents. A long, distressing and exhausting process. I had thought at the time to air it in 'another place' but decided against doing so.
On the maternal side it was a case of 'let nature take its course'; months of gradual decline followed by four or five days and nights of edging toward the final rotating door. I slept on the floor, witnessed and attended each and every step down, until the last breath. But that was her wish - no interference - no speeding up or slowing down. In her case euthanasia would have been against her wishes.
On the paternal side it was different; the doses of morphine were gradually increased to ease the pain, the reluctance to let go. Witnessing the fear of passing was the worst. Someone once said, and I paraphrase, that females often gladly relinquish the fight knowing they can do nothing, but men become afeared to the last that they can't change a thing.
Each and every passing is uniquely different; it is a point in time and to make it of one's own choosing is a luxury that perhaps few of us can afford. To make a choice however, voluntary euthanasia, when of sound mind, that one's life should be terminated when the struggle is clearly over... yes, I'd go with that.