Krugman says the same thing that I've said here many times: that the USD is never going to go away as a unit of measurement. And all Bitcoin amounts to is another means to move around USD-denominated value. Nobody says, "please give me 0.029827 of Bitcoin", rather they ask for some amount of USD in Bitcoin.
USD may not disappear from the global market, but it may cease to be used as a benchmark and a currency used globally in trade transactions. For example, if China's economy overtakes the US, why shouldn't the yuan become the reserve currency?
I fully accept that the bitcoin (given its current and possible future value of $1 mln.) could be used for international settlements, but in the equivalent of the same dollars. That is, they might say, "give me $1 mln.", but they'll pay that amount in bitcoin (13.88
BTC).
Indeed, the idea that "Bitcoin will replace the USD" is... idiotic. That idea simply makes no sense--it's like saying TCP/IP is going to replace the USD.
Idiotic? We'll see who's truly idiotic when international settlements between countries start using bitcoin.

Or when Iran starts charging oil carriers in bitcoin instead of USD.
Definitely worth reading if you are interested in the long-term role of Bitcoin in the world, and how it fit in to human value transfer.
Regarding this, there are doubts between people, but on the scale of groups of people (countries), it seems probable (to a greater extent).