[ABetaMale]

Amyloid-targeting drugs have not failed. There are currently three — aducanumab (phase 2 trial successful, one of two phase 3 trials successful), donanemab (successful phase 2 trial), and lecanemab (promising phase 2 trial which technically failed but which showed a 64% Bayesian posterior probability of being at least 25% better than placebo) — that have shown a tendency to a roughly ~20-35% reduction in the pace of cognitive decline compared to placebo.

That said, amyloid is very upstream in the causal chain, which begins about 15-20 years before clinically detectable symptoms. It's believed by some (including myself) that amyloid therapy would be preventative at that early stage, but you won't do much better than a ~35% slowdown at the stage Alzheimer's is typically diagnosed. At that stage, tau therapy is more promising, since it's the more proximate cause of neurodegeneration.