It is also possible that the amyloid hypothesis is somewhat on the right track, but just need to target the right part of the pathway. For example no one has designed any clinical trial yet according to this article recently published in the journal Nature (a Gal3 inhibitor drug, belapectin, currently exists):
Galectin-3 promotes Aβ oligomerization and Aβ toxicity in a mouse model of Alzheimer’s disease
https://www.nature.com/articles/s41418-019-0348-z