J. R. Hicks, in the 1937 article in which he proposed the IS–LM analysis, argued that Keynes’s General Theory did not represent a major break with the classical tradition.

In particular, he maintained that the main insight it contained was into the conditions existing during a depression or a deep recession. Was this claim valid?
Have Keynesians contributed anything further since Keynes’s General Theory? Does the above claim apply to the various Keynesian models?
Does the modern classical approach provide an adequate analysis of the economic conditions in recessions and depressions, or does the profession still need the Keynesian approaches for its analysis?