"economics based on maximization-with-equilibrium. We imagine an economy consisting of rational, self-interested players, and suppose that economic outcomes reflect a situation in which each player is doing the best he, she, or it can given the actions of all the other players. If nobody has market power, this comes down to the textbook picture of perfectly competitive markets with all the marginal whatevers equal."This is clearly incorrect. First, classical authors, meaning those that followed the surplus approach (from Petty to Marx, including Quesnay, Smith and Ricardo) did assume that economic agents were rational and self-interested and they also believed that the economy could be represented by equilibrium outcomes. And they clearly were not neoclassical, meaning they did not believe that supply and demand determined long term prices (natural prices as Smith and Ricardo referred to them, or prices of production in Marx's terminology).
If profits were higher in a particular sector, capitalists would try to gain from those opportunities entering the industry, and in the process would lead to a uniform rate of profit. Market prices, determined by supply and demand, would gravitate around the long term equilibrium prices that were determined by the technical conditions of production, and the previously given real wage (by conflict), in modern parlance (on the issues raised by the Labor Theory of Value, and Sraffa’s solution just check other posts in this blog).
More importantly, there was no mechanism (even in the case of those classical authors, like Ricardo, that accepted Say’s Law) that implied full utilization of labor, capital or any particular means of production. Wage flexibility did not lead to full employment. The hallmark of marginalism is the notion that supply and demand determines simultaneously the equilibrium long term prices, and that price flexibility leads to full utilization of resources, something that the capital debates have demonstrated long ago it cannot be done. In this regard, Krugman decides (because it must be advantageous) to follow those that he criticizes, and remains oblivious to both logic and empirical evidence. If he wants to be coherent with his Keynesian ideas, he should get rid of the notion of a natural rate of unemployment (