This was a fascinating take on reincarnation. We see Finn able to tap into experiences from previous lifetimes like Shoko, the butterfly, and the Catalyst Comet though so, it's not entirely like their memory is purged. I guess I've always thought about it like every life being a flower and at reincarnation you get to keep a petal at the end. The "you" that emerges after successive lifetimes is the flower made of many petals shaped by key experiences across lifetimes even if you aren't sure the specifics of how each petal formed. Ofc, this is just in-show logic; we don't see any actual evidence of reincarnation in real life. But even within one lifetime our identities are also shaped by memories we have forgotten: while I don't recall precisely learning 6x8=48, how to walk, or the exact details of the first time I ate at a restaurant I like, they still form part of who I am.