This gets insanely complicated legally and ethically, because there has to be independent proof of loss of mental competence (which happens when...?) - but paradoxically, assisted suicide legislation to date assumes a person is competent to make the decision for themselves.
Clearly both can't be true. So at best you end up in a brief window where someone can be assumed to be competent enough to understand their situation while also understanding that the competence may disappear in the near future - possibly by the time they've travelled to a clinic - and someone else will be making a life-ending decision on their behalf.
There also has to be no suspicion that anyone will benefit from accelerating the suicide option - which is not always as straightforward as it sounds, and would ideally have to be investigated explicitly on a case by case basis.
My mother had middle-stage Alzheimer's at 90, although - perhaps mercifully - cancer killed her before Alz could.
We had issues with power of attorney, because she was very clear she wanted to go home and live on her own, and it was absolutely obvious that she wasn't able to do so without endangering herself.
Aside from the emotional heartbreak of having to tell someone over and over every time you meet that they can't have something they really want - because she couldn't remember a conversation from minute to minute, never mind day to day or week to week - there were also a formal legal process involving two oversight bodies (local services, and the national Court of Protection) to make sure that keeping her in a home against her will was actually in her interests.
It would have been far harder if she'd left a living will saying "Please make arrangements for assisted suicide when it gets to this stage" and then apparently changed her mind as the disease progressed to the point where there was no possibility at all of mental competence.