> Back of the envelope calculations suggest that even modest propulsion technology should be sufficient for a single technological species to spread throughout the galaxy in a geological instant

I think this is where it all went wrong. Overly optimistic assumptions from atomic age thinkers about how technology was going to overcome all obstacles in the near future based on their recent life experience. The thinkers seriously underestimated the amount of technological progress needed to produce an extremely complex machine that operates indefinitely with no outside support whatsoever in an environment where any error can be fatal and there is a constant stream of hazards to contend with.

You might be thinking: "So what, it just means it takes a few centuries longer to get started, in Geological terms that is nothing.", but the problem isn't that it is hard, it is that it makes the trip significantly harder than simply building orbital habitats in your home solar system. Once you have that, what is the point of spending vast amounts of resources in trying to colonize a mostly unknown star system tens or hundreds of light years away?

Sure there are always people who will want to climb Everest, but this problem is so big that it's unlikely that one person or even a small group will be able to undertake it. The resource investment is simply too colossal. It would be much harder to climb Everest if you had to convince the entire population of London to come with you.

I don't think they were wrong about that. I think what they were wrong about is that we could maintain our civilization at the same level they experienced long enough to make significant progress. We dropped the ball in less than two generations and it's yet to be determined if we can pick it back up.