Again, we're talking about a civilization that is colonizing the galaxy here. They would presumably have habitats and people strewn all over the star systems th

Again, we're talking about a civilization that is colonizing the galaxy here. They would presumably have habitats and people strewn all over the star systems they colonized. What do you see wiping them all out entirely so regularly that 100M years as an average is so optimistic? And it doesn't even need to be a single civilization. You only need a civilization to exist capable of launching a new colonization venture every 100k years, or whatever the average rate they assumed was.