Immediately obvious to anyone who has experienced serious impairment is that the low end of the scale needs to account for levels where your functionality continues to drop (i.e. having difficulty staying conscious or coherent, as in chronic fatigue) and where your willingness to continue suffering drops or ends (not wanting to do maintenance tasks, considering suicide, actively pursuing it). Of course it goes farther in the other direction too, in terms of what variety and how much you can manage: in particular, whether you feel confident making advance plans or regular commitments. That's usually the first thing to go with any disability. It is useful to consider this end because people may have specific goals that require juggling resources to obtain.
Quality of life has two aspects, internal and external. Internal ranges from miserable through neutral to happy. External ranges from a drain on resources to breaking even to contributing. Life is justified and enjoyable when you are neutral to happy, neutral to contributing a majority of the time. When life becomes a burden to self and others, it is not right to demand that people continue, although they have a right to do stick it out if they choose. It's the in-between parts that people argue over the most, where someone is happy but a burden, or miserable but useful.