Mankind’s destiny is to explore and expand. Not retrench.

Is it, though?

One of the main points of the book is that the limits to our colonizing space may not be technological know-how. It's that we inhabit bodies and have brains that were optimized through untold millions of years to live on earth. We have a pretty wide habitat range, since we're a fairly flexible species - but it's not unlimited.

One key example they explore is human psychology. Many humans tend to do poorly being confined in small spaces with other humans with no opportunity to get away. Some humans do well at it, but many many humans aren't going to be able to do it. Past experiments involving the types of conditions likely to be encountered in long-term space travel and/or colonization, like the Bio-Dome, reveal the need to carefully select participants for their ability to withstand those types of conditions. As I think you intimated upthread, explorers and pioneers have a certain spirit and personality. So if you're doing an expeditionary or exploratory mission, like every other space mission we've done, you can select for those traits.

But unlike exploration, colonization involves a second generation - and beyond. Instead of having participants that have self-selected or been chosen for their fitness for those conditions, now you're just getting random humans. Not entirely random, of course - but people that have not been screened for their suitability for living in small enclosed spaces with other people. Some of them, likely many of them, will not be able to do it.

While a mission can make sure those people aren't stuck in space conditions with careful screening and replace them with people who are, a colony cannot. Which makes the colony doomed to fail. Unlike any earth conditions, a space colony with any technology we're likely to have in the next century or so is not likely to be suitable for average humans with average human psychology to live in.

And that's before we get into what happens to babies as they grow up in 1/6 or 1/3 earth gravity. As in, we don't know if they can develop normally if they're not in earth-normal gravity, and develop their bone structure and musculature without it.