This is actually not Keynes, it was originally Lenin who predicted that in the future people will live like we do on retirement, because machines would do everything for us.
You have to always measure the productiveness agains the goods you want to get. Today we are incredible efficient in getting the goods that would consume 90% of your time 100 years ago. But once we satisfied those needs, new appetites arose, so we now not only want shoes and bread, but also iPhones and intercontinental flights.
I'd look at some of the religious communities, like ultra-Orthodox in Israel, who are able to live for the whole month on very modest welfare, supporting large families of about 10 children. But no car, no phone, no Internet, no plasma, no fancy vacations, games, gym memberships, Starbucks or online shopping etc.
We are incredibly good at inventing new needs.