In regards to dementia, it isn't at all clear that that will necessarily lead to information-theoretic death. We don't have a good enough understanding of dementia to know if the information is genuinely lost or just difficult to recover. The fact that many forms of dementia have more or less lucid periods and periods where they can remember who people are and other times where they cannot is all tentative evidence that the information is recoverable.

Also, this argument isn't that strong an argument. This isn't going to be substantially altering whether or not it makes sense to sign up by more than probably an order of magnitude at the very most (relying on chance of violent death and chance that one will have dementia late in life).