I have (had) two relatives succumb to dementia (Alzheimers) before they passed.

In one case, that person reverted back to the memories which were at the time of my early childhood. We reconnected in a way that shed off the later traumas for both of us, and while I still could not love that person, I appreciated them for who they were (or thought they were) at that time. And I could grieve their passing.

The other person was pretty much dead to everyone anyways, so yeah, once their memories were gone, they were already dead to us.