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.