Even now, we might be less than 50 years from independent colonies, if lower costs for space launch finally "take off" with Musk.
This implies that if there is some common reason (e.g. a physics experiment with unexpected outcome) that exterminates budding civilizations, it ought to have already happened (and we were lucky) -- or it is something big enough to blast a whole solar system.
Edit: It would be interesting with percentage chance evaluation of the possibility that the Shuttle project doomed humanity to extinction...?
Surely whatever improbability would be diminished because of this?
Since we don't see galaxy-spanning, highly advanced civilizations, there seems to be some combination of factors that prevents planet-bound dirt from turning into them. If those factors are primarily after our stage of development between dirt and galactic civilization, that's Very Bad News, because we shouldn't expect to get extraordinarily lucky. If those factors are primarily before our stage of development, that's reasonably good news, because we've already made it past the hard part.
Seeing bacteria isn't as bad as seeing the ruins of a civilization just past our stage of development. But it does move the probability-mass forward more than if we just found dirt.