In 1936, when Keynes wrote the "General Theory," the world's key economic problem was unemployment. There were too many people and not enough jobs.
Today, the world's key economic problem is sustainability. There are too many people and not enough resources.
The solutions to the two problems differ sharply, despite the formal resemblance (too many people, too few x). In Keynes' day, demand management on a country-by-country basis could correct the market and policy failures that caused unemployment. Today, demand, supply and technology solutions must be coordinated on a global basis to correct the market and policy failures that threaten the planet.