But then who decides if the patient is of sound mind?
Professional psychiatrists, the family and the individual... like seriously. Do you think the process is easy? Go on, brew some coffee or tea and then sit to search how certain countries handle dignified end of life.
See how that process is usually a long series of tests, conversations and verification. It's not "let's pop down to the doctors and get grandma euthanised". It's an insane amount of beurocracy and a lot of time before you even get close to a due date, and even then you can cancel at any time.
I think people need to get more informed about this issue, because you can't exactly form an opinion without seeing how something like that can work IRL, when it's been proven to work IRL. More people are accepting of it and a lot of pain and heartache will be spared because of it, because at the end of the day it's about dignity.
Forcing people to continue when they can have no life to continue, when they are a financial and emotional burden on their families and unable to actually grow and live, like an actual person, like me and you. To force them to just sit there, like a potted plant, like a doll you put away on the shelf when you return to your life. That is selfish, that is undignified.