There are some difficult philosophical questions here. If someone has an ailment, that is causing their mind to think euthanasia is the answer, but if they had the treatment to the ailment, they would no longer think so, are they of sound mind? I'm not sure where the line is on that, and i think its far from obvious.

Agreed. Likewise though, there's an ethical dilemma the other way: Is it acceptable to force treatment on someone?

These are not easy questions and while I personally believe that assisted dying legislation will prevent a lot of needless suffering, I also believe there will be people effectively killed by this who could have otherwise had fulfilling happy lives but couldn't see the path out of the woods or were unable to make that journey.

Feel it is likely you are over estimating modern medicine’s ability to judge if someone is of sound mind.