Krugman on why Greece shouldn't consider "Grexit" unthinklable

Paul Krugman writes again on the macroeconomics of Greece and the relevant considerations (History Lessons for Euro Debtors 07/15/2015):

Now what? If Greece still had its own currency, the case for devaluation would be completely overwhelming at this point. What this means, in turn, is that everything — the ongoing economic disaster in Greece, the bitter divisions within the euro area, the perplexity of even the best intentioned policymakers — flows from the supposedly insuperable technical difficulties of going off the euro.

Can this possibly make sense given the extremity of the situation?