... The lessons of the crash era are that these megabanks have grown beyond the organic controls of capitalism. They were so big and so systemically important in '08 that the government could not let them go out of business.

This alone was an argument for breaking them up. The banks emerged from '08 with the implicit backing of the federal government. They became quasi-state entities, almost immune to failure. Not just Bernie Sanders worried about this. Voices as diverse as Louisiana Republican David Vitter and Krugman's own New York Times editorial board have argued for hard caps on bank size.

What's happened in more recent years, with LIBOR and the money-laundering scandals and Forex and the London Whale episode and so on, is that these firms also proved too "systemically important" to regulate and prosecute. They grew too big not only for capitalism, but for criminal law.

When a company is not only too big to fail, but too big to prosecute, it's too big to exist. Krugman may believe otherwise, but he shouldn't pretend that others – including his own paper – don't have legitimate concerns.