3. I’ve focused on the “galaxy” somewhat arbitrarily. Spreading throughout all of the accessible universe would take a lot longer than spreading throughout the galaxy, and until we do it’s still imaginable that some species from outside our galaxy will disrupt the “stable galaxy-scale civilization,” but I think accounting for this correctly would add a fair amount of complexity without changing the big picture. I may address that in some future piece, though.
3. I’ve focused on the “galaxy” somewhat arbitrarily. Spreading throughout all of the accessible universe would take a lot longer than spreading throughout the galaxy, and until we do it’s still imaginable that some species from outside our galaxy will disrupt the “stable galaxy-scale civilization,” but I think accounting for this correctly would add a fair amount of complexity without changing the big picture. I may address that in some future piece, though.