There is plenty of data that shows that people are bad judges of their future opinions. “If X happened to me, I wouldn’t want to live anymore” often turns out to not be true.
That makes it questionably whether consent years or even months ago implies consent now.
And yes, that is very problematic in cases such as Alzheimer’s where people cannot consent now.
These are untested treatments with unknown impacts. Consider playing Russian roulette with the patient. The risk isn't the same but the outcomes have the same range. From nothing to death.
But they could give consent in advance.
If this horrific disease progresses to the point where ... I give my consent for ... Subject to final approval from family member/doctor/whatever.