Well, even a better argument to bring those factories to the US. Why not develop the know-how on manufacturing and improve automation in the US rather have China lead there.
Of course there are a lot variables in the above. As time goes one automation gets better. You can buy cheap robots for some common operations, and a good engineer with good CAD can run various automated analysis to ensure fit and then export to the robot and build even a single part cheaper than the human - amortizing the cost of the robot over thousands of different single parts made this way. However as the widget gets more complex you reach the point where humans are needed. In some cases you just have humans to take the parts off of one machine and put them into the next, but it is still humans. We can automate even that, but often the robot to do that would cost more than a human for 10 years.