It hasn't been ruled out, but as Derek Lowe mentions, there is a good amount of genetic evidence that increased cleaving of APP and thus more amyloid-beta leads

It hasn't been ruled out, but as Derek Lowe mentions, there is a good amount of genetic evidence that increased cleaving of APP and thus more amyloid-beta leads to earlier, faster and more severe Alzheimer's. And certain mutations in the amyloid-beta producing APP-cleavage machinery leads to familial dominant Alzheimer's, where the carriers will almost certainly develop early Alzheimer's. So there is not many ways around APP/amyloid-beta having some role in Alzheimer's pathology.