[ivalm]

I agree that it might be fundamentally impossible to make meaningful macroeconomic predictions (because it is self correcting). But this is one of the arguments why macroeconomics might fundamentally not be a science (kind of like the art of writing fiction isn't a real science, although there are many commonalities/themes/etc one can study).

Re your third paragraph. You make many claims which may or may not be correct, but at the end of the day nobody has any kind of track record of prediction long term inflation rates of GDP numbers. This means that there is little value in counterfactual claims (if we did x then y would happen). Every study (such as your allusion to '29) are explanative, not predictive. Explanative studies have relatively little to do with what makes something a science (prediction).

Finally, this is not to say that there is absolutely nothing meaningful in economics. Surely if you arbitrarily set tax rate to 100% or 0% you would have negative consequences. Similarly, if Fed changes current rates dramatically, this would have consequences. It's just for "normal" changes in policy we have basically no predictive power other than something like nearest-neighbor search in historical observation.