How does physicalism (physical monism) explain what that loss is?
The question, to me, seems... kind of absurd. How do we know that someone is dead? How does, for example, an EMT or a doctor decide that a patient is dead?
They don't pray. They don't light incense and seek the answers from souls and spirits. They decide a person is dead based on physically detectable signals. Do they have a pulse? Or brain activity?
We know someone is dead first and foremost because of physical things. The physical aspect of death isn't some great mystery, it's the most immediate way we know someone is dead in the first place.