I'm saying that the treatment pathway for each of those conditions is very different. Chances are somebody who has end-stage cancer—as a previous witness mentioned—has a much clearer picture that that their condition is indeed irremediable and that they are indeed going to die anyway or in the foreseeable future, even though that part is different now. They likely didn't experience the same kind and degree of stigma, discrimination and failure of social supports that somebody with a mental health problem or illness did.
I can absolutely defend treating them equally in isolation, because the contexts that lead people to that point are very different.
There's also the added piece that mental illness will, by definition—even if you don't lose decision-making capacity—inform the decisions you make. We can't ignore that context.