Lets just say if some people did not have money, they would be as useful as a wet tissue.
If I didn't have money, I could offer my skills to barter for other goods or services. This is what money fixes, it creates a field where everyone can play on.
The problem is when all your skills and what you have to offer gets automated by machines that are 100000% more efficient and cheaper than you.
This would make no sense on a larger scale. Machines exist so that humans could switch their activity to something more interesting/rewarding. If all men would one day become unemployed, then there would be no market for the goods machines produce. So it is a self-balancing system with a feedback loop...
The self-balancing mechanism only happens decades ago, when you still had lots of demand for the whole society and lack of production capacity. When most of the production can be done by a few companies, majority of people will be out of job and income
It is true that if all the other people are unemployed, then how to sell the products that robots made? No worry, government will borrow a bit to hand out as food stamps and social security and keep majority of people living a just OK life. While robot companies and banks took large amount of resource and build their heaven island somewhere in Caribbean
What is the purpose of overcapacity? There is none, so it is profitable for governments that people should be employed to the full by any means (besides, employed people are less prone to social unrest). There is still much room for making the elites' lives better, even in the Caribbean. So no worry for the majority of humanity...