Not the parent poster, but they could have added uncharted Oort cloud object collisions, extra-solar system objects, quasar/interstellar jets, nuclear war, runa

Not the parent poster, but they could have added uncharted Oort cloud object collisions, extra-solar system objects, quasar/interstellar jets, nuclear war, runaway climate change, mega volcano/caldera eruptions, bio-engineered runaway viruses, and probably a few other potentially life ending events for humanity that would bring that 'billion' number down by several orders of magnitude for likelihood of our extinction.