I have gradually become convinced by mountains of evidence from many quarters that the Keynesians are right-- we are in a demand constrained economy.
One of the problems I think is that economists and politicos tend to always be fighting the last war. In the late 1970s to early 1980s -- the last time the economy seemed this systemically bleak -- the problem was insufficient capital available for investment combined with high inflation. This resulted in "stagflation," something classical Keynesians thought impossible, and thus resulted in a widespread discrediting of classical Keynesian and economic liberal thought.
Thing is... in the late 70s the supply siders may well have been right and the Keynesians wrong. We were in a supply limited economy, and stimulus and other liberal policies just made everything inflate.
But now I think the situation has reversed. Today looks a lot more like the 1930s, albeit not quite as viscerally nasty. There's a huge amount of capital available but few good investments. Wages are stagnant, savings are down, pay and wealth inequality are utterly massive, etc.
So today I think the Keynesians may well be right again. Hopefully it'll come in the form of something less destructive than a war. I'd personally suggest a massive program to electrify all transport, huge investments in both renewable energy and next-generation nuclear power, a medical "war on aging," and a campaign to establish a permanent human settlement on Mars. All that taken together would probably end up costing about what WWII did, and so might be enough. The war on aging would have the added follow-on benefit of helping with our demographic problems. (As would a more liberal immigration policy.)
Meta question: perhaps there's a new insight here... do modern industrial economies actually tend to oscillate between demand constrained and supply constrained failure modes? Perhaps the Keynesians overcorrected for the great depression, resulting in the 1970s, and now the supply siders have overcorrected for the 1970s resulting in today. In that case maybe we oscillate between periods when Keynesian demand-side policies work and periods when Friedman-esque supply-side policies work.
Edit: the reason I don't think we're hurting as bad at the street level today as we were at the depths of the Great Depression is simply because we've grown so far beyond subsistence level. Today the amount of economic pain required to cause visceral physical pain among large numbers of people in the developed world -- starvation, etc. -- would be quite a bit larger than what it took in the 30s. To lead to that kind of thing we'd need the 2008 financial crash times ten at least, combined with a major political collapse and some kind of technological train wreck. A mega-sized solar flare or a sudden "oh crap" worst case peak oil cliff coming to pass might do it.
But compared to a non-slump timeline -- compared to what should be happening given the technological growth we've experienced -- we are in a depression. We should be colonizing Mars, conquering aging, pretty much doing all the things I suggested as stimulus. Instead we are building apps like Yo while people go bankrupt in underwater houses with underwater student loans. (Not blaming the authors of Yo either... during the depression a lot of people took up whittling sticks and other little time-passing pursuits for lack of good work to do.)