Two deployed airbags in a moderate (but not life threatening) crash likely cost $1000 each to replace, in addition to whatever other damage has been done to the vehicle.
First, actually adding airbags does not add thousands of dollars to the cost of a car. Hundreds, maybe.
Second, I will gladly pay extra for such a highly effective safety feature.
Third, I'm quite certain the medical costs saved by airbags every year will more than offset the cost of installing them on every new car. And that's just cold calculation, without bringing the moral value into it.
