So what did happen? Keynes offered an answer: it is, in fact, possible for economies to suffer from an overall lack of demand. Other people had said things along these lines, but Keynesian economics put it front and center.

...

So let me summarize: we had a scientific revolution in economics, one that dramatically increased our comprehension of the world and also gave us crucial practical guidance about what to do in the face of depressions. The broad outlines of the theory devised during that revolution have held up extremely well in the face of experience, while those rejecting the theory because it doesn't correspond to their notion of common sense have been wrong every step of the way.

Yet a large part of both the political establishment and the economics establishment rejects the whole thing out of hand, because they don't like the conclusions.

Galileo wept.