Currencies are goods like any other goods and the people who own them have divergent purposes. When Krugman writes "we want [a monetary system] to facilitate transactions and make the economy as a whole rich", he's assuming a lot. Obviously, some of the people who own bitcoins right now are not interested in facilitating financial transactions to the detriment of their net worth. So, for people who want that facilitation, bitcoin isn't getting the job done, but the people who are using bitcoin as an investment are getting just what they want. Those two conditions could flip fairly quickly down the line, but that's how these things go. But for Krugman to just assume that only one of these is an acceptable condition just begs the question.