Whilst I cannot speak for
@nheather, I know in my own personal experience of dementia that my father had. 4 years of slow (and sometimes not so slow) deterioration of his physical and mental abilities until he was little more than a husk in a chair in a dementia care facility.
There is no quality of life in that existence and there is no dignity in it either.
Add to that the knowledge that my father himself had on many occasions (before dementia took hold) expressed his own personal belief that he "did not want to end up vegetating in chair" and in no way would he wanted to have "experienced" the last few years of his life the way he did.
He passed away a couple of months ago and while some may think it sounds callous, I can honestly say I am glad for it. No longer is he suffering, slowly eroding away in a chair as a shell of his former self.
Dementia is an insidious disease, by the time it has truly "taken hold" they are no longer themselves and no longer have the capacity to choose euthanasia even if the option had been available.