It's a measurement on an arbitrary scale. Nothing I've said is news to anyone who designs safety critical systems. I'm certainly not saying that safety isn't important or that we can't assess it. What I'm saying is that placing a number on that assessment will always stray into the realm of politics in a way that physics and mathematics never does. It lulls ignorant people into the belief that something is safe or not safe. They feel safe because they've been told it is safe or vice versa. Physics doesn't care if you feel safe.
It's notable that contemporary safety standards such as ISO 26262 generally avoid numerical assessments, for the reasons outlined above.