Keep in mind that there's ALWAYS a tradeoff in everything, even how you eat and exercise, you eventually come up with diseases caused by your diet or exercise.
Example? If you eat a lot of fish (salmon, sardines, etc), you'll have lower risk of heart attacks as observed in studies BUT there's another thing that many don't mention - you also increase the risk of bleeding stroke.
Smoking - you'll be less likely to have Parkinson's or Alzhiemer's if you smoke but you're much more likely to have a heart attack or lung cancer.
So, we need to look at the whole picture. If smoking protects Parkinson's, why not start smoking? Well, here's the thing - you're much more likely to die of a heart attack caused by smoking than to die of Parkinson's if you never smoked.
So, even with cures, it can kill you in totally different ways. For example, aspirin can reduce heart attacks but it will increase the risk of bleeding.
I imagine that if everyone eats fatty fish, more people would die of strokes and you'd be bombed with suggestions to cut down on eating fish. That's exactly what's happening with studies.