When it comes to my end of life decision, I looked upon my pets as examples. I noticed anytime one of my cats had reached a time in his/her life that living became too difficult due to very advanced age/medical issues, he/she would just stop eating and drinking. No amount of coaching would change their mind.

At first it was so difficult to watch until a Vet reassured me that was nature way of letting ago. The animal was choosing breathing over eating/drinking because the animal could no longer do both. And there were options to make sure the animal wasn't suffering during this process. It was like pet hospice on an accelerated level.

Getting to the last stage of the disease may taken a long time.
A very long time!
Like in the case of Ronald Reagan who died ten years after being diagnosed with Alzheimer's.
Not only will that require tremendous physical stamina from the caregiver but also an equally formidable financial stamina!
How many people can do that?
The important thing is to establish at which point to put into effect the terms of the living will.
I think that the point is when the person so affected is no longer able to care for himself and require constant care 24/7 and have no chance whatsoever of leading a "normal" life.
Here, the most important factor is that the quality of life is completely gone and to carry on living is no long worth while.
A fast ending would be had by carrying out voluntary euthanasia most probably by an injection of a massive dose of nembutal.