Countercounterevidence for 3: what are the assumptions made by those models of interstellar colonization?
Do they assume fusion power? We don't know if industrial fusion power works economically enough to power starships. Likewise for nanotech-type von Neumann machines and other tools of space colonization.
The adjustable parameters in any model for interstellar colonization are defined by the limits of capability for a technological civilization. And we don't actually know the limits, because we haven't gotten close enough to those limits to probe them yet. If the future looks like the more optimistic hard science fiction authors suggest, then the galaxy should be full of intelligence and we should be able to spot the drive flares of Orion-powered ships flitting around, or the construction of Dyson spheres by the more ambitious species. We should be able to see something, at any rate.
But if the future doesn't look like that, if there's no way to build cost-effective fusion reactors and the only really worthwhile sustainable power source is solar, if there are hard limits on what nanotech is capable of that limit its industrial applications, and so on... the barrier to entry for a planetary civilization hoping to go galactic may be so high that even with thousands of intelligent species to make the attempt, none of them make it.
This ties back into the hypotheses I left out of my post for the sake of brevity; I'm now considering throwing them in to explain my reasoning a little better. But I'm still not sure I should do it without invitation, because they are on the long side.
It's sticky sweet candy for the mind. Why not share it?
We have a sample of one modern human civilization, but there are some hints on how likely it was to happen.
Major types of hints are:
Data for:
Data against:
To me it looks like life, animals with nervous systems, Upper Paleolithic-style Homo, language, and behavioral modernity were all extremely unlikely events (notice how far ago they are - vaguely ~3.5bln, ~600mln, ~3mln, ~200k or ~600k, ~50k years ago) - except perhaps language and behavioral modernity might have been linked with each other, if language was relatively late (Homo sapiens only) and behavioral modernity more gradual (and its apparent suddenness is an artifact). Once we have behavioral modernity, modern civilization seems almost inevitable. Your interpretation might vary of course, but at least now you have a lot of data to argue for your position, in convenient format.