There's a pretty convincing argument that a long-term surplus will result in inflation if it pinches aggregate supply; Keynes was looking at a few short years of the business cycle, but there are longer term trends that will assert themselves if you use Keynesian policy indefinitely
Keynes acknowledged this, though I can't remember how clearly he agreed with it, and argued it was no reason not to act in a crisis, but was MUCH more conservative when advising the Allied government on setting up the Bretton Woods system of international finance for the long term
