You work at a power station, so you probably already realized that the real currency of the world is the kWh(or megajoule). Since 2010, solar and wind have been able to produce more energy during their lifetime than the total amount of energy needed to create/ship/install the product.
Robots build everything and we hypothetically have unlimited energy(we just need to use energy to build more energy production units).
That is known as a post-scarcity economy. That is what Keynes was beginning to point towards. Think of "Star Trek" or the Culture book series.
Anyway, I would argue that people work for 40-hours per week because it is the amount of work that keeps them sane. Work less and you feel lazy. We will probably always be doing about 40 hours of work, even in retirement.