One has to remember that cars that were designed to be without power steering usually has 5 turns of the wheel from one side to the other, while cars with power steering only has 3, again usually. So a car that was designed to have power steering and looses it, is not really comparable to cars designed to be without one, and is a real pain to drive, especially in the city.
You might be confusing a few things - older steering columns were very rigid (and thus "strong"), but were also a fairly major concern in frontal or rear collisions: they would essentially impale the driver if hit at the right angle.
Modern steering columns have a telescoping construction that allows them to collapse in the event of an impact so that this isn't a big of a concern.
It doesn't matter whether it is a servo or a human that turns the shaft, the design loads are a function of the weight of the car and the front/rear weight distribution as well as of whether or not the front wheels are driven or idlers.
