I don't think unknown unknowns are what I'm thinking of here. In this case the argument involves a very specific risk, and sometimes a very specific lower bound

I don't think unknown unknowns are what I'm thinking of here. In this case the argument involves a very specific risk, and sometimes a very specific lower bound for its probability. For example, in the hash collision case the argument says that it's not worth worrying about the (known) probability of a software error due to an unexpected hash collision because it's dominated by the (known) probability of a comparable error due to cosmic radiation. (The former probability can be calculated using the birthday paradox formula, and the latter has been characterized experimentally in different kinds of semiconductor chips.) This kind of argument doesn't rely on the idea that there are other risks that we can't identify or quantify. It's about comparing two failure modes that we did think of, in order to argue that one of them is acceptable or at least not worth further attempts to mitigate.