Question

when John Maynard keynes said, "in the long run we are all dead," what problem of classical economics was he pointing out

Answer 1

Problem of classical economics pointed by John Maynard keynes:

He was complaining about economists who believed that unemployment would be reduced or eliminated after the recession if the market is not somehow stimulated or steered in right direction. He agreed that this strategy can work in long run, however, in short run, it would pose some problems due to people’s nature of hoarding money and banking preferences.  

The classical economists believed that in the midst of a great depression and recession, the government should not do much and the economy would recover on its own which may take decades for such to happen.