We already give people the right to reduce the amount of time they are conscious
We already allow people the right to be unconscious during an otherwise painful medical procedure. Euthanizing someone who is in permanent suffering is effectively just a scaled up version of this. If someone prefers being unconscious to suffering from some level of pain, then euthanasia is effectively the same thing, just on a longer time horizon. Since there's not effectively a difference between death and being rendered unconscious for the rest of your life.