I agree that selves diverge as memories diverge. But if one imagines being obliterated and then moments later duplicated (into two copies), would you think of these as new subjects? If you knew they were going to suffer, would you fear the suffering as something that you will experience or pity them as one might a person who is not oneself? The question of fear or pity is the best way I can get at this distinction between a self as some collection of memories and personality traits, and the subject as the locus of experience. My father had Alzheimer’s so I know what you mean, but by the same token I think the the experiences he had late in his life when his memory had entirely failed were still happening to the same subject. They were the future experiences of the young man he once was, even though much of what made him him in an identity sense had disappeared by then.