There is a disturbing fact that would bother me more if I thought the rise of technological civilization were at all likely: the fact that we see no sign of "them" is evidence such societies have short lifespans.

Two possibilities -- maybe technology contains within itself the certainty of self-destruction, or maybe there are things we don't know about (such as other dimensions or time travel or what-have-you) that advanced societies eventually find and disappear into, leaving our cosmos.  Of course it is also possible that everyone sooner or later holes up underground in a virtual reality.