Some conservative critics prove Krugman wrong by the following syllogism:
Nobody is always correct.
Krugman claims he's always correct.
Therefore, Krugman is wrong.
It's valid as far as it goes. Krugman always responds by listing the times he was wrong. He stresses (over and over and over) that when that happens, he changes his mind.
But most people do that as best they can. What makes Krugman special is that he will back out of beliefs that everyone in his tribe still holds. For example, Noah Smith, and Brad DeLong will criticize DGSE models, but they would never doubt the neoclassical conviction that economics can only be studied by analyzing equilibrium.
Krugman follows a typical progression, he changes his mind first on his blog, then in more prominent places.
And it remains true that Keynesians have been hugely right on the effects of monetary and fiscal policy, while equilibrium macro types have been wrong about everything.