Krugman was saying Bitcoin could NEVER work as a monetary system because it's designed to be deflationary. I.e. People would hoard instead of spend
This is why gold never worked as a monetary system either

Great insight !
. . .
It inflates initially; however, that inflation decreases to nothing over time (and, in fact, becomes deflation as the rate whereat coins are lost or made unspendable overtakes their production).