It pretty much is settled fact. There were some hold-outs, but we recreated the conditions of the Great Depression, and then we had a crash as large as the Great Depression. Countries pursued more or less Keynesian policies in response, and the countries that pursued the most Keynesian policies recovered the fastest. It's as close to an experimental test of Keynesianism as we're ever likely to see, and it came out in favor of Keynesianism.