Honestly, the practical thing that bothers me is that once someone's gone, they can't say yes or no, so finishing their work with AI just creates stuff they never actually approved. I'm not sure how to argue it better, but that feels off to me.
The other side might have a point about it being practical, honestly I can't argue that well. But a dead artist can't sign off on anything, so finishing their work with AI just feels like putting words in their mouth. That's my gut, even if I can't prove it.