The idea of knowing you're going to get dementia sometime in the next decade is interesting. But I don't think it's straight-forward as to whether or not knowing is a good idea. It's also a little amusing to take the idea to its logical extreme: What if it becomes a societal norm to know? Would public venues allow soon-to-be dementia patients line cutting priority etc? Think amusement parks, transit reservations, hot…
We already do that. We think you may get cancer or dementia, we deny you insurance converge and proceed to drain you of opportunity
Most developed countries (and many less developed countries) have some government coverage for cancer or dementia.
In New Zealand we can also pay for premium private insurance - although I don't think that helps much for chronic conditions.
Dementia will often still drain your bank account in New Zealand because the government here will take all your equity to pay for nursing home fees. But you do get looked after a bit when you have nothing, so there is that. You can avoid having all your equity taken but it does take some planning, some costs and some effort (e.g. give equity to children more than 7 years before you lose your marbles, e.g. use a family trust, e.g. if married it is very important to ensure your will limits equity damage to one if the other falls ill).