What to reply to keynesians when they say "stimulus is good because look what happened in 1929"?

As you might have guessed, this has been gone over ad nauseum…

It’s all right here:

And here’s a nice little snippet:

And of course there’s plenty of lectures on this…

The Great Depression playlist

And if you need more on why WWII didn’t “get us out of the Depression”, see here.