And this isn’t even something made up for the particular case of assisted suicide. The idea that someone with mental illness cannot make legitimate choice has been around in the legal system for a while, with mental illness leading to reduced sentences, and even being the basis of acquittal in many situations.
There is mental illness and then there is competence, they are related but not the same. Complicating things further in this case would be things like reactive mental states (grieving etc) which might for a time manifest like mental illness but tend to be not diagnosed.
> and a vast majority are thankful of it
A statistic that (assuming it's true) suffers from the obvious self-selection bias of people who chose to jump off a bridge knowing there was a net to catch their fall.