When a topic only has a limited number of experts, those experts become gatekeepers.
Those gatekeepers directly or indirectly control research funding.
Gatekeepers necessarily harbor biases, some right and some wrong, about how the field should progress.
For Alzheimer's, some gatekeepers were conflicted and potentially directed the field in the wrong direction. Only time will reveal AB42's true role.
It's easy to find fault in Alzheimer's.
It's harder to see the general solution to the gatekeeper problem, i.e., how to allocate resources in areas with limited experts.
The research went awry in Alziemer's due to fraud but its being funded at a reasonable level, a level many with Long Covid or ME/CFS or Fibromylgia would be very happy to see but doubt will ever happen.
Naturally, the far more terrifying and inexorable disease that is incurable and robs people of their entire personality and will affect most of us to some extent (dementia, if not Alzheimer's specifically) by the end of our lives gets more funding and attention, as it should. The way Alzheimer's has been researched and funded is diabolical, though, but you might pick any other of 200 serious progressive neurological disorders that are underfunded and underrepresented over... CFS. CFS isn't even fully accepted as a syndrome at this point - long COVID is probably more accepted as a real thing by practitioners at this point than CFS.
Isn't long covid just CFS that can be attributed to Covid?
If you accept that multiple viruses can cause "long <virus >" syndromes, of which long covid is just one example, it's plausible that CFS is really a cluster of syndromes, one category of which is these post viral syndromes. We just can't pinpoint the virus behind it every time because most viruses haven't been studied as much as Covid has.
That's also why Alzheimer's can take so long to develop. It's just one aspect that we've chosen to focus on because it's more clearly noticeable, but it cannot easily be treated in isolation from everything else. If it was, it would regress quickly without fixing the root causes.
Possibly the most likely possibility?
Karl Herrup has a terrific book on the topic How Not to Study a Disease — The Story of Alzheimer’s from MIT Press (2021, ISBN 9780262045902). He did not win many friends but I think he is right.
The consensus now is that many factors contribute to the heterogeneous diseases we now call Alzheimer’s.