I understand the "detect deadly progression but no cure" problem; this was the same rationale people used when Huntington disease could be verified in diagnostics. Many people don't want to know, but some want to know, in particular as you can manage some things here or there - diet affects many things, for instance, even aside from metaboic genetic defects. And for any (molecular) therapy at a later time you need to understand the molecular basis to some extent. Some things can be found out via trial and error (vaccination and before) but for some disease that can not work. Alzheimer is quite complex.