The arguments against the sovereign rights of an adult over their own bodies are shaky at best.
If the desire to die at a time of ones own choosing, rather than endure months of destructive, eventually fatal illness automatically puts a person into the category of mentally unsound there is an imposition of a personal morality upon another person who is experiencing or is going to experience a great deal of pain. To me that is a great injustice.
I have never believed that guilt is an adequate reason for a person to choose to live and endure great suffering. If a person chooses to seek treatment, if a person chooses to simply live as long as they can, those efforts should be encouraged and supported. Positive goals of every kind should be supported, but saying to a person with a terminal illness 'Please don't seek an assisted death because you will hurt your loved ones.' is a negative. It binds a person with obligation, rather than love. A grandmother choosing a course of chemotherapy and radiation to see a grandchild be born or some other milestone is a beautiful, positive aspiration. We have to give the same respect to those who choose a comparatively comfortable, planned ending.
There was one writer who commented that she would never support physician assisted death because she feared for the lives of her disabled friends and colleagues, and people like them. This author was not herself disabled. As someone who meets criteria and doesn't like the word 'disabled' this is an attempt to remove agency from people who were not gifted with whole and hearty bodies. The risk for abuse and misapplication of physician assisted death is definitely higher for people with disabilities just by having more interactions with physicians than the average person. That does not mean that someone else gets to make decisions about how we live or end our lives.