Sustainable capitalism is fundamentally based the on balance of power between a capitalist's need for labor and labor's desire for a certain quality of life. Laborers, using the power of their numbers demand payment, amenities and regulations that help create a stable consumer base for the capitalist to sell to. This drives the economy. When robotics can replace essentially all labor the general population will be removed from the economic cycle leading to oligarchy or significant economic changes that would not primarily be considered capitalism.