> A possible chink is that civilizations just don't do that -- carpet colonize the whole galaxy, or fill it with their probes to the extent that we seem them all over the place

This assumes that there are gazillons of alien civs yet somehow they all come to the same decision. That's much less likely than gazillions of alien civs all make different decisions and it only takes a single one to make the replicating probs or try to colonize.