See https://www.astralcodexten.com/p/in-defense-of-the-amyloid-h... for an IMO compelling argument that amyloid really is the underlying cause. The theory is that amyloid buildup causes misfolded tau proteins which cause the cognitive damage. So reducing amyloid in people already suffering from Alzheimer's doesn't do much because they already have excessive tau; at best it might slow down the progression.