A reader writes:
We naturally want to fill in the blanks in these scenarios, and I think this one is easy. Why did they go extinct? In the time case, a billion years seems like ample time for something out of our control to happen. In the space case, it seems plausible that extinction was the result of the rapid expansion caused by us. If you mentally devise different answers, I think it is easy to make the space case seem more admirable.
Another points out a potential flaw with Hanson and Grace's scenario: "The history of evolution teaches us that the population of species that remains in one place and does not diversify or learn to adapt to different climates will die much, much sooner than the species that expands as far and fast as it can."
The Dish 