Just what will, indeed could, be done with this information?
Assuming they can correctly predict Alzheimer's, so far as I know there is currently no effective treatment. In which case, about the only advantages I can see are a) allows time to make decisions and tidy things up; b) avoids treatment for other possibilities. And, just how long before any real deterioration would anyone wish to know this diagnosis?
But what if they wrongly predict Alzheimer's and the issue is actually a treatable condition? I have seen someone with a thyroid conditions more or less diagnosed with dementia and, possibly, Parkinson's. I am aware of people with Pernicious Anaemia who have been diagnosed with brain and mental conditions. Both of which can improve massively with treatment. (Though there are still major arguments about the exact choices of treatment.)