Thing is, as Steve Keen among others has pointed out, liberal New Keynesians like Krugman are every bit as responsible for that Dark Age as their rivals at Chicago and Minnesota. Case in point: His widely-cited 1989 paper on Income Elasticities and Real Exchange Rates. The starting point of the paper is that floating exchange rates have not, in general, adjusted to balance trade flows. Instead, relative growth rates have roughly matched the growth in relative demand for exports, so that trade flows have remained roughly balanced without systematic currency appreciation in surplus countries or depreciation in deficit countries. Krugman:
The empirical regularity is that the apparent income elasticities of demand for a country's imports and exports are systematically related to the country's long-term rate of growth. Fast-growing countries seem to face a high income elasticity of demand for their exports, while having a low income elasticity of demand for imports. The converse is true of slow-growing countries. This difference in income elasticities is, it turns out, just about sufficient to make trend changes in real exchange rates unnecessary.The obvious explanation of this regularity, going back at least to 1933 and Roy Harrod's International Economics, is that many countries face balance-of-payments constraints, so their growth is limited by their export earnings. Faster growth draws in more imports, forcing the authorities to increase interest rates or take other steps that reduce growth back under the constraint. There are plenty of clear historical examples of this dynamic, for both poor and industrialized countries. The British economy between the 1940s and the 1980s, for instance, repeatedly experienced episodes of start-stop growth as Keynesian stimulus ran up against balance of payments constraints. Krugman, though, is having none of it:
I am simply going to dismiss a priori the argument that income elasticities determine economic growth... It just seems fundamentally implausible that over stretches of decades balance of payments problems could be preventing long term growth... Furthermore, we all know that differences in growth rates among countries are primarily determined in the rate of growth of total factor productivity, not differences in the rate of growth of employment; it is hard to see what channel links balance of payments due to unfavorable income elasticities to total factor productivity growth. Thus we are driven to a supply-side explanation...Lucas or Sargent couldn't have said it better!
Of course there is a vast literature on balance of payments constraints within structuralist and Post Keynesian economics, exploring when external constraints do and do not bind (see for instance here and here), and what channels might link demand conditions to productivity growth.  Indeed, Keynes himself thought that avoiding balance-of-payments constraints on growth was the most important goal in the design of a postwar international financial order. But Krugman doesn't cite any of this literature.  Instead, he comes up with a highly artificial model of product differentiation in which every country consumes an identical basket of goods, which always includes goods from different countries in proportion to their productive capacities. In this model, measured income elasticities actually reflect changes in supply. But the model has no relation to actual trade patterns, as Krugman more or less admits. Widespread balance of payments constraints, the explanation he rejects "a priori," is far more parsimonious and realistic.
But I'm not writing this post just to mock one bad article that Krugman wrote 20 years ago. (Well, maybe a little.) Rather, I want to make two points.
First, this piece exhibits all the pathologies that Krugman attributes to freshwater macroeconomists -- the privileging of theoretical priors over historical evidence; the exclusive use of deductive reasoning; the insistence on supply-side explanations, however implausible, over demand-side ones; and the scrupulous ignorance of alternative approaches. Someone who at the pinnacle of his career was writing like this needs to take some responsibility for the current state of macroeconomics. As far as I know, Krugman never has.
Second, there's a real cost to this sort of thing. I constantly have these debates with friends closer to the economics mainstream, about why one should define oneself as "heterodox". Wouldn't it be better to do like Krugman, clamber as far up the professional ladder as you can, and then use that perch to sound the alarm? But the work you do doesn't just affect your own career. Every time you write an article, like this one, embracing the conventional general-equilibrium vision and dismissing the Keynesian (or other) alternatives, you're sending a signal to your colleagues and students about what kind of economics you think is worth doing. You're inserting yourself into some conversations and cutting yourself off from others. Sure, if you're Clark medal-winning Nobelist NYT columnist Paul Krugman, you can turn around and reintroduce Keynesian dynamics in some ad hoc way whenever you want. But if you've spent the past two decade denigrating and dismissing more systematic attempts to develop such models, you shouldn't complain when you find you have no one to talk to. Or as a friend says, "If you kick out Joan Robinson and let Casey Mulligan in the room, don't be surprised if you spend all your time trying to explain why the unemployed aren't on vacation."
 "In practice there are many channels linking slow growth imposed by a balance of payments constraint to low productivity, and the opposite, where the possibility of fast output growth unhindered by balance-of-payments problems leads to fast productivity growth. There is a rich literature on export-led growth models (including the Hicks supermultiplier), incorporating the notion of circular and cumulative causation (Myrdal 1957) working through induced investment, embodied technical progress, learning by doing, scale economies, etc. (Dixon and Thirlwall, 1975) that will produce fast productivity growth in countries where exports and output are growing fast. The evidence testing Verdoorn's Law shows a strong feedback from output growth to productivity growth."
 Who was it who talked about "the phenomenon of well-known economists 'rediscovering' [various supply-side stories], not because they’ve transcended the Keynesian refutation of these views, but because they were unaware that there had ever been such a debate"?