Of course, the search for life in the Universe is still in its infancy. What is hypothetical often times turns out to be imaginary--that is, it is unlikely that reality will comform exactly to our expectations. The numbers I came up with describe one possible scenario. Until we have more data, assigning specific likelihoods to various numbers would be premature. However, I think it is highly unlikely at this point that the original 1960s estimates for the number of galactic civilizations (millions) are correct. Reason: we should have seen some sign that they exist, they would be closely spaced, etc.

I was assuming, again, just for fun, that other civilizations will be roughly like ours in that they will dominate their biosphere in a haphazard before settling into a pattern of sustainable living. Those that don't make this transition, including our current civilization, will face decline and eventual extinction. If the number of civilizations is low to begin with and a good fraction of them succumb to HWM (home-world mismanagement), an even smaller number achieve interstellar flight.

The people above who said that it is bad science to extrapolate from our civilization are the same people who say that the laws of physics and chemistry universality makes the chances of life more likely in the Universe based on extrapolation. This type of position is not contradictory, but a legitimate question is: where is the line in exobiology that separates reasonable extrapolation from unreasonable extrapolation given the universality of natural laws?