I don't think this accurately represents the arguments of the Keynesian economists. Krugman, to pick an example, calculated that high unemployment rates would cause us to miss out on at least $3 trillion of wealth creation, and he called for about $1.5 trillion in infrastructure investment. The government would capture perhaps 25% of the restored growth in taxes, so the federal government would come out at least $750 billion behind. But in theory, America as a whole would be wealthier. (All numbers from memory.)

The actual "stimulus" consisted of about $700 billion, of which maybe $350 billion was actually spent on things that Krugman thought would help the economy. He predicted that this was far too small to have any real impact, and that we would therefore spend years in a weak economy. (We later spent additional money on unemployment insurance; I don't know how this affects the models.)

So while you could very easily disagree with Krugman's theory, his models accurately predicted the results of the stimulus: A minor, temporary boost that would be insufficient to actually fix anything. So at least one high-profile Keynesian called it correctly, although you might argue that he just got lucky.

Personally, I don't know enough economics to have an informed opinion on my own. But we have at least 15 million Americans who should be creating wealth, and who are currently sitting around doing nothing. I'd love to see that get fixed, especially because the last 20 years of Japanese history suggests that if we alternate between rounds of weak stimulus and austerity measures, we might be stuck in this economy for decades.