Robots are used because automation is cheaper than human workers.
If humans couldn't be hired to perform tasks because of robots, then the companies using robots would go out of business and/or the economy would spiral out of control towards collapse.
A business only works because people can pay into it, thus a constant equilibrium (only interrupted by state interference) exists in which employers can only offer services so long as employees exist to pay for said services.
Therefore robots cannot replace humankind because to do so would cause humankind to be unemployed, leading to the collapse of said robotics companies and the rehiring of humans.
This does not take into account changing markets, it only points out that we don't have to worry about robots.