Uh, Krugman's whole point is that the capped supply is inherently deflationary so the incentive is to hang on to bitcoins, ie, not the best attribute of a currency.
There are plenty of legitimate arguments that can be constructed against the idea of an inherently inflationary currency too.
In short, empirically, wages and prices are sticky. We assume it has something to do with humans being stubborn, but don't really know why. However, until our imminent replacement by the kind robots, currencies must move if people won't.