as our society becomes ever more wealthy and machines replace humans, we still have to work, work, work to have any value or self-respect -- so the masters say.

It's about two basic variables: Workpower and money.
Workpower is needed for creating goods.
Money is needed to distribute goods.

1. In slavery, a person is connected to workpower, but shut out from the money. They can not buy, they are no consumers.
2. Today, people are connected to both, workpower and money.
3. When all the work is done by robots, people are shut out from work. Where do consumers get their money from?

If money were in the future still connected to workpower, then those people would be neutralized as consumers and the economy would break down. Therefore workers loosing jobs to robots would have to be compensated with a social safety-net. The money they get from social sources would keep them inside the economy.

For example:
Government gives money to consumers.
Consumers give money to corporation.
Corporation gives money to government.


EDIT: So, capitalism could lead to replacing workers with robots, would could in turn lead to a shift to socialism to take care of the people who can no longer give money to corporations.