It may not change much as far as researching a treatment or cure goes, but it may help with other stuff, like being better prepared for the future, like organizing a health care worker or getting family members to help out or to look out for you, and other stuff, which you might not be able to do at a later stage of Alzheimers.
While there aren't any cures yet, certain treatments and lifestyle strategies may slow its progression, keep quality of life as long as possible and stuff like that for as long as possible. (And the sooner you start with that, the better)
More information is not always better for the patient. If you could detect the disease 5 years before symptoms began, there are certain psychological harms that come with that knowledge. These must be balanced against the things you mention about "slowing" the disease (unclear if any treatments do much for a given individual) and planning your future. You talk about quality of life, but quality of life declines the MOMENT you learn that you have a progressive, incurable, disease that will slowly rob you of your mind. It's not clear at all that knowing about your disease earlier is actually better for anyone.
I understand the concern about anxiety about their impending condition, but medical providers must not paternalistically decide to withhold a diagnosis from patients, at least not in all cases.
If I got an early diagnosis, it would motivate me to get my affairs in order to lessen the burden on my family and check off some bucket list items before it's too late. Don't rob me of that opportunity.
Before ordering the test, ask patients "If you were going to get Alzheimer's, would you want to know?"
While there aren't any cures yet, certain treatments and lifestyle strategies may slow its progression, keep quality of life as long as possible and stuff like that for as long as possible. (And the sooner you start with that, the better)