Keynes — and others who later elaborated on his work, like Hyman Minsky — taught us that although markets are usually self-correcting, they occasionally enter destructive feedback loops in which a shock to, say, the financial system scares business and consumers so badly that they hoard money, which worsens the damage to the system, which further persuades other economic players to hoard, and so on and so forth.Many of us economists believe Keynesian policies have been successful, and that more would have been better, but politics doesn't judge outcomes relative to a counter-factual scenario. That is, the argument that things would have been far worse in the absence of a policy isn't a winner, even if it is correct. Unfortunately, that means future policy makers are likely to draw exactly the wrong lessons, and do even worse next time (at least on the fiscal side; central bank independence gives monetary policy some space to follow academic rather than political views).
In that situation, the role of the government is to break the cycle. Because businesses and consumers have stopped spending, the government breaks the cycle by spending. As clean as that theory is, it turned out to be a hard sell.
The first problem was conceptual. What Keynes told us to do simply feels wrong to people. “The central irony of financial crises is that they’re caused by too much borrowing, too much confidence and too much spending, and they’re solved by more confidence, more borrowing and more spending,” Summers says.
The second problem was practical. “What I didn’t appreciate was the extent to which we only got one shot on stimulus,” Romer says. “In my mind, we got $800 billion, and surely, if the recession turned out to be worse than we were predicting, we could go back and ask for more. What I failed to anticipate was that in the scenario that we found we needed more, people would be saying that what was happening showed that stimulus, in general, didn’t work.”
Furthermore, as Paul Krugman explains, the economics profession (or at least some parts of it) isn't playing an entirely helpful role.