Let’s stipulate that O’Neill was assuming some breakthough in propulsion that was going to drop access to space by orders of magnitide. Maybe even make it as cheap as a transcontinental flight.
20 years to build a colony is still outrageous. In that time, we would have had to:
design an entirely new class of spaceship
Build lots of them.
Design and build mass drivers on the moon
Prospect for, mine, and transport millions of kilos of material to the mass drivers.
Build in-situ construction facilities for recieving said mass and shaping it into a colony.
Outfit the colony with an infrastructure. Electrical, water, sewage, roads, etc.
Transport up thousands of people, build dwellings for them, give them jobs, provide initial foodstocks, etc.
In the 60’s, we already had experience with large engineering projects. The Boeing 747 took five years from first concept to first flight, and it was just a relatively simple extrapolation of well known construction techniques and concepts.
I suspect O’Neill picked the smallest number he thought he could plausibly get away with in order to drum up support for his ideas. It’s a lot easier to make people enthusiastic over something that might happen in their lifetime than 100 years in the future.
Well, I suspect you’re right about that. As I said, 20 years is an extreme lower bound, based upon a massive and compounding effort to build up a colony; this was at the time that the environmental movement, impelled by Rachel Carson’s Silent Spring (and those crying Italian Indians) and continued with the Club of Rome’s report “Limits of Growth” were were predicting massive resource depletion and worldwide famine in the Eighties. For O’Neill and his followers, the colonization of space wasn’t just about exploration or expanding scientific knowledge, but moving the mass of humanity and their industrial activities off the planet’s surface; to this end, they were planning on a huge, single-minded effort by humanity in achieving this goal. At the very least, this was politically naive, but if we were to make such a concentrated effort, utilizing every scientific and intellectual resource available, something on the close order of 50 years would be achievable. Instead of thinking about the 747 (a commercial project demanding an economic return on investment) think in terms of the Manhattan Project or the Minuteman development.
Of course, the Club of Rome’s most dire predictions have not (yet) come to pass, and as time goes on we see that those claims were, if not completely invalid, misguided and exaggerated. The impetus to expend such resources and effort on a speculative and far-reaching program has never come to pass, and the notion of transporting billions of people off-planet into orbiting habitats is a pipe dream short of erecting some tolatarian world government. The people who end up populating space will be those who are born there.
In any case, this did not happen, and the odds of it happening within our lifetimes are miniscule to say the least. But this does not mean that, should we find the political will to do so, that the technical and logistical problems could not be addressed and resolved.