You repeatedly say this as if this is a physical law, but you have no evidence at all for it.
Yes, liquid at 160ºF can cause burns if it is held against the skin at that temperature for a period of time.
What you forget is that there is a certain amount of energy associated with a cup of coffee, and particularly with a small spill. The spilled coffee will begin to rapidly cool as it gets in contact with the clothes, skin and air. The lower the initial temperature, the less energy will be transferred to the skin before the temperature is below a burning point.
Another way of putting this: If you fall into a vat of coffee, it doesn’t make much difference if it’s 180º or 190ºF, you’ll be burned pretty severely either way. But if a tablespoon of coffee drops on your arm, that 10º difference may well be significant. The cooler tablespoon will more quickly reach a safe temperature, and the actual damage done to the skin will be lessened.
So you can be sure that, even if it’s still possible to scald yourself at the new temperature, the number of burn victims almost certainly decreased.
In the view of the suit, McDonalds believed that their need for higher temperature outweighed the elevated risk to customers. The new temperature that the suit caused now decreases the risk to customers, without severely putting McDonalds at a disadvantage. That’s the point, not that the risk to customers has been completely eliminated.
