Was Keynes Right?

Tyler Cowen on problems with the theory underlying contemporary Keynesian economics:

Aggregate demand macroeconomics works in many cases and it almost always “works” (predicts well) when the macro forces are pointed toward destructive ends. We are not sure why it works at all, or if it always works, and yet we see a great fervor of belief in it and a demonization of those who are skeptics.

Am I misunderstanding this? Right now macro forces are indeed pointed toward destructive ends, aren’t they? So if AD macro almost always works in those cases, doesn’t a great fervor of belief in it make sense even if we’re not 100% sure why it works?