It will be a major oops moment when we will find out that dementia is caused by some weird kind of bacterial or prion infection. And the whole moral explanation of it ("go read your damn books and talk to smart people or you will get Alzheimers!") will turn out to be same bullshit as moralistic obesity explanations ("just lazy and can't control yourself!") recently turned out to be.

One big thing that changed in these decades is cleanliness simply due to better cleaning chemicals. Which certainly caused a reduction of infections of all kinds.