You are correct that Kant based autonomy on the concept of dignity and human value. They are two slightly different concepts. Autonomy is the rational capacity to make one's own decisions, but benevolence is not necessarily aligned with autonomy. Some authors do say that it is malevolent not to respect autonomy and benevolent to respect it. In their classical sense, these two principles are very different: a person's autonomy can be infringed while doing good.
Instead, I want to point out here that in dementia, autonomy is lost. The autonomy exercised in advance medical directives or in everything that is a previous guide is a way of representing autonomy in the future. That autonomy cannot be fully exercised because the person is not aware of what is happening. At that point, we are in the register of benevolence.
The state does not prevent people from being well by preventing them from having access to medical assistance in dying; the opposite is true, in fact. My argument is, rather, that if we permit medical assistance in dying, we run the risk of encouraging people to believe they no longer have value, they are bad, they stink—that's the word you used—when they suffer from advanced dementia and are sometimes lying in their own excrement, unfortunately. We clean them, we look after them, we care for them. That care is beneficial because the previous autonomy no longer exists...