This (see vid. link below) is what I would call pretty conventional thinking about SETI and the likelihood of local aliens. It seems to me it makes a seriously faulty implicit assumption about the likely lifetime of intelligent civilization, about which more below. Also, it doesn't take into account the fact that the greatest likelihood of the origin and development of planetary life to the point where the rise of a technological species is relatively likely (say 5 by from start to emergence, on average) to have been well in the past, due to the fact that the majority of main sequence stars formed within 2-3 billion years of the peak of star formation, which was 9 +/- 1 b.y. ago. This point may not seem obvious, but it is well nigh inescapable. So if any of those civilizations figured out how to do long range planning and colonization, they would have started such programs literally billions of years ago on average. Again, I think the implicit assumption that the lifetime of civilizations is likely on the order of 10,000 years or less is just plain wrong. Species are short-lived, although on Earth the time scale for big animals is more like 1 million years. But advantageous adaptations tend to survive much, much longer. Think of photosynthesis. Body plans that include circulation, respiration, etc. Consciousness itself. Very advantageous. The first organisms that evolved these things are long extinct and looked nothing like living organisms. But these traits survive and develop. I contend that technology-level intelligence is a breakthrough adaptation, like photosynthesis or eukaryotism. Fundamental. And likely to survive, in some form, far longer than the species that first developed it in any given biosphere. Add to this that a technological civilization will, almost by definition, stop being subject to the vagaries of chance and will be able to direct its own evolution, to ensure the survival of its mental development and technology. People shy away from thinking about that, but history doesn't care about our sensitivies. What in the way of adaptation is possible and will benefit some segment of a population, will happen, at least some of the time. It's a law of evolution.
My conclusion is that, usually, or at least some of the time, once technological intelligence gets firmly established, it becomes a permanent feature of a biosphere, and has the added benefit of freeing that biosphere from the constraints of a single planet of origin as its only habitat. Eventually, technological civilizations that manage to evolve from a natural biosphere begin to expand into a galaxy, and beyond. And on astronomical timescales, this takes far, far less time than the time that has been available since it first became plausible that technological civilizations could arise in any give swath of space, such as our Galaxy. Think this through. The inevitable conclusion is that life may be common; even advanced complex life. But technological intelligence is extremely rare. Because if it had arisen just a few times (to allow for failures) in the past in our Galaxy, it would still be here and would have likely spread throughout the entire Galaxy long since. To my mind, this is the essential insight of the so-called Fermi Paradox. (Which I prefer to think of as the Intelligence Rarity Paradigm).
