I'll ignore the moralistic Stross material, both because that is wise and because Krugman takes pains to emphasize the positive nature of his argument. The tentpole of that is that there is no theoretical floor for the exchange value of BTC. This is roughly true, but I don't think it entails the disastrous consequences that excite so many BTC antagonists.
Even if it's difficult to imagine hyperinflation of USD (although I'm not quite so confident as Krugman in the Fed's dollar sink policy: if this is not a new policy why didn't it work in the 1970s; if it is new why shouldn't we expect it to change?) we've seen hyperinflation of many other currencies, a number of which were held by more investors than currently hold BTC. If such hyperinflation was disastrous, then it was mostly for those who held the affected currency, and mostly in proportion to their holdings. No tinpot dictator has decreed the use of BTC; if you think they're overvalued then just sell them.
Of course if there is eventually a giant terminal sell-off of BTC for reasons, investors might just choose some other set of cryptocoins, some of which might have an appropriately theoretical floor. Would Krugman care to opine on the positive fate of those theoretically worthier assets?