This is great news. 60% of Dementia is due to Alzheimers whilst remaining 40% is roughly split between Vascular dementia (dementia due to mini-strokes) and Dementia with lewy bodies (a histopathological finding).

Unfortunately Alzheimers is largely a pathological diagnosis (ie. it requires a brain biopsy usually post mortem to diagnose) and I didn't dig up the british study this is largely taken from to see where they thought the decline was coming from but it is very good news and should be welcomed!