I'm not familiar with dementia other than what I just read in the American Medical Assoc. Encyclopedia. And that was not a lot. So I am going to try and address your last statement according to what I just read.
Are you saying then: Without the memory of what happened in a previous life, if such exists, we have nothing? And therefore, to be reincarnated offers us nothing for we will (if such exists) not remember anything from the past life? Thus, the self (in the past life) is dead!
If I understand you to be saying this, then I whole heartedly agree. For if I could not remember having lived in this life, when I reach the possibility of another incarnation, then what good would it do me? For I would have no recollection of this and therefore would not know that I had lived now. But, should I read of "me in the now" at some future date, then all I would have was that someone named "me" had once lived and I would not know that it was me in a former life.
So the value of reincarnation would be nill.