The 20th century economy theory developed under conditions of more or less constant growth. But how much can economy grow, and what happens when we cannot double car sales, dentist services and financial paper exchange every 28 years (under 2.5% annual growth) anymore?

Keynesians are generally right how to revive growth in a crisis - if it is possible. Nevertheless, the reality is a bit more complicated than Keynesian toy models. Debt problems are that huge that respecting them 100% leads to a nanny global state for creditors: they would be getting their cut regardless of whether economic exchange would be growing, or could be growing at all. With the growing weight of interest entitlements, productive economy would have hard chances for growing even if energy and other resources were practically limitless - that is the interpretation of Keen, Hudson of how the classical economy (including Adam Smith) has to be applied today. And if we are close to "growth" limits of some key resources, then cyclical economy and technological progress are just sweet stories before a Real Downer.