I don't buy the fraud explanation as the full explanation. Other areas of medicine (stem cell) has had bigger incidents of fraud on top of other major headwinds, and still has made more progress.
Fraud is everywhere and we still move forward in most arenas.
In ALZ and the plaque cartel the fraud was foundational and the overwhelming source of funding for research was tied to supporting that hypothesis. The big issue, even if you have a competing theory, is that the diagnostic criteria relies heavily on the plaque and presence of indicators. So you get a group of people who have elevated plaque and MCI, but many people have elevated plaque without MCI, and just as many people have MCI without elevated plaque.
So if your cure is targeting something different but the group of people you have are selected from this cohort of maybe afflicted people then it's really hard to get a significant result. Plus you tend to be dealing with old people, that have other health issues that MCI isn't causing to get any better.