I guess what you describe above sounds sensible. There are two practical problems with it.
One is that you are, by all evidence of your comment, an individual with above average inteligence and planning ability. I have a suspicion if we leave you on a deserted island in a few years you might be halfway on your way to your own iron age. Unfortunately not everyone who is smithen by devastating chronic suffering is as lucky as you are in that department. The rules of the society has to work for everyone, not just the smart ones. For each and everyone who thinks through what you did there are hundreds and hundreds who can’t and won’t.
Then the other problem, society wise, is that typically by the point someone reaches the end of the road and their illness is deemed incurrabble they are not well. Long illness weakened them, maybe they are paralysed, or blinded. What you describe is hard, but even harder if one does not have all their physical faculties. And that is very sad, but unfortunately that is the main case legislation like this aims at.
At least that is how I think about when i think about the purposes of such law.