How much of an issue is this? How often do steering systems actually mechanically fail? I would guess its no higher than the number of people who have a heart attack or seizure while driving.
Teslas have enough sensors to know that the car is not doing what it told the steering to do, and presumably can then brake immediately. So in a complete steering fail, even at high speed, you are likely way better off than having a human drive.
And of course if steering can fail, what exactly would a human be able to do about it?