"They are not rare enough to treat the idea with casual contempt ."
Where you treating the idea that way? I didn't notice. What I did notice was your statement that by avoiding a collision we might be safe for a couple centuries. I hadn't thought we were at the point of being in peril every 200 hundred years. I don't think we yet know how often we're imperil and from an historical perspective humans have not been disastrously impacted by one of these anywhere near that often. As far as I know.
BSB