I know a lot of legitimate research supports various versions of the amyloid hypothesis, but I don't buy that these likely fraudsters had minimal impact.
You said that Lesné was "not that highly cited". But his main fraudulent paper was cited 2,300 times, making it the fifth most highly cited Alzheimer's paper since 2006! [1]
Berislav Zlokovic's likely fraudulent papers were cited 11,500 times! [2]
It's hard to imagine these highly papers didn't redirect at least some scientists to do pointless followup studies. Of course, in the counterfactual world the scientists might still have been doing pointless studies, but we'll never know...