--because Alzheimer's disease is a spectrum, from mild forgetfulness to more than that but not dementia yet. It's Alzheimer's disease.
My argument for broadening it to the brain at large is that the pathology of Alzheimer's is actually a combination of things. There is some Parkinson's in there, and Lewy body, and some small stroke components, and amyloid and other changes in the brain cells. So it's actually a complex disease with bits of different pathologies.
Some patients will have a Parkinson-like course. Others will have a more traditional dementia, a typical Alzheimer's course. And others will fall early and have incontinence because of the stroke component.
That's why there is also a pragmatic, pathological reality check. Alzheimer's is a complex of different causes, and the brain approach will pay off better in the long run than just a disease-specific approach.
I never thought I would say that, but it's true.