i see no reason to assume (a core, a priori assumption of the paradox as elaborated by Hart) that technological civilizations would constantly expand outward from their home planets--which is the major burden of my argument.
I don't necessarily think that technological civilizations would "constantly" expand outward, or even that they would "usually" expand outward. The problem for me is that apparently "none" of them have ever expanded outward.
All it would take is one alien species with the desire (for whatever reason), and a technology similar to ours, and they could have colonized the galaxy within a million years. And if their tech was better, it might be faster and cheaper.
Even if most lack the desire or the economics to do it, you would still think that at least one
race would have tried it. If only one race tried it every million years, it still should have happened 65 times just since the dinosaurs died. And our galaxy has been around for billions
I guess the astounding thing to me is that no galactic races, zero, none, have left any visible mark within our galaxy in billions of years. That seems rather unlikely even if we postulate that most lack the wherewithal or enthusiasm for galactic expansion.
There are many terms in the Drake equation which can drive the result to zero, but there are also reasonable arguments for many terms which leave the results in the hundreds or thousands.
Apparently something is
driving the result (visible evidence of colonization) to zero. But we don't know what it is. It might be a combination of things.
At the moment (based on what is visible to our technology and our way of thinking), it appears that for some reason, all the conditions (including economic and motivational) required for visible colonization of a galaxy have never come together even once in this galaxy, even over billions of years. To me that's still a paradox because it doesn't jive with what seems reasonable based on the way I would guestimate the Drake variables.