I think it's a little more optimistic than that. Recently there was a Kaggle competition that analysed enzyme make-up of saliva of animals in order to predict epileptic seizures (I might be mincing specifics here but the point remains). So while it's true that we won't be able to treat some of the things that a fitbit etc. highlights (bad decision making, cancer, etc.) it can definitely potentially be used in preventative diagnoses and early detection warning systems. What an age we live in indeed!