> If the robotics are feasible their rise should be exponential, not linear

That totally does not follow. Robots can be feasible and their payback curve can still be linear or sublinear.

Take mining rare earth metals, for instance. The problem with mining rare earths isn't that it takes so much human labor. The problem is doing it without poisoning the surrounding 30 square miles. Replacing the driver of the earth mover with a robot doesn't fix the chemistry at all, nor (I suspect) does it significantly affect the economics.