There are plenty of things that could destroy all human life on Earth and not destroy all human life on all planets.
Also the argument taken to an extreme is straightforward. We know that massive catastrophes have caused mass extinctions that wiped out most life on Earth. This is clearly visible in the fossil record. Ergo, one of these is certain to happen again. In the extremely long term, the Sun will get hot enough to kill all life on Earth in a couple hundred million years. So Humankind is doomed if we do stay on just Earth for the very long term.
There's also the side argument that "right now" appears very close to making it possible. If Starship and the attempt at reusable rockets were to disappear there's nothing guaranteeing the progress of space technology. It's easy to see a future where we do some more footprints on the Moon and maybe later some footprints on Mars, but no permanent off-earth colony is ever established. Eventually we give up on creating such bases because they're deemed too expensive and "robots do it better" as our robotic technology gets better and better. The technology of how to do reusable vehicles is eventually forgotten over several generations and then we never leave Earth again. (Eventually resulting in our destruction.) This is a "why not now?" argument.
Also the argument taken to an extreme is straightforward. We know that massive catastrophes have caused mass extinctions that wiped out most life on Earth. This is clearly visible in the fossil record. Ergo, one of these is certain to happen again. In the extremely long term, the Sun will get hot enough to kill all life on Earth in a couple hundred million years. So Humankind is doomed if we do stay on just Earth for the very long term.
There's also the side argument that "right now" appears very close to making it possible. If Starship and the attempt at reusable rockets were to disappear there's nothing guaranteeing the progress of space technology. It's easy to see a future where we do some more footprints on the Moon and maybe later some footprints on Mars, but no permanent off-earth colony is ever established. Eventually we give up on creating such bases because they're deemed too expensive and "robots do it better" as our robotic technology gets better and better. The technology of how to do reusable vehicles is eventually forgotten over several generations and then we never leave Earth again. (Eventually resulting in our destruction.) This is a "why not now?" argument.