Since in order for an economy to thrive it requires people that are able to make and spend money. Take all the jobs, by replacing them by robotics, you soon run into higher unemployment, which can lead to less spending, generating a vicious downward spiral. Sure, robotics can perform tasks that most humans could not possibly do, such as the space arm, working in conditions and being able to purport amazing feats of strength and precision. However, what if we had a robot to do almost anything, such as depicted in Will Smith, IRobot, what task will be left for humans, except to build more robots, which eventually the robots could do for themselves, which sooner or later suggests the Doomsday effect again.