Because she will pounder in old memories that never existed in the first place. Instead of cherrying the current times. Past moments of long gone time will be elongated beyond their actual existence.

Imagine a time where a small moment is actually a smaller amount experienced then the remixed one. At one point you will have more memories of fake events, and more emotional beats for said fake events than real ones.

AI videos showing you a second life if you just had chosen a different path.

People will be depressed from it.

Because the mother's joy should hinge on the actual photo of the actual event of the actual person, not a fictional imaginary event or fake childhood of their child. The experience of being human should be tethered to reality.

Because elderly people who never had a connection to tech or sci-fi will have no feel for, and therefore no resistance to the kind of trickery that seems fun but is ultimately corrosive. Cheerfully meddling with the artifacts of memory at that life stage is corrosive.

Why do you assume her joy is the sad part?

Memory already declines with age. AI slop threatens to warp those memories further.