That’s the mentality that told Magellan not to set sail or he might fall off the end of the Earth.

That mixes up exploration and colonization. Magellan (and others) discovered lots of places that no one chose to colonize. Because those places were completely horrible for long term human habitation.

The authors’ conclusions were based on a few points. First, it is impossible to overstate how unsuitable for human life any space location is. Everything is radioactive, poisonous, often incredible sharp and abrasive, and it lacks almost everything we need to survive.

Second, colonization requires babies and kids. We don’t know if we can reproduce without gravity. We don’t know if babies can develop properly without gravity, or with reduced gravity. You obviously can’t get a one year old on a treadmill for four hours a day to keep their body from deteriorating. And we can’t replicate gravity on a surface of a planet. And there’s no ethical way to find out - unlike adult astronauts, babies and kids can’t ethically consent to experimental or test efforts.

And finally, even trying to colonize space is probably more dangerous to humanity than going without a Plan B - because it increases the chances we lose Planet A. Historically, national efforts to compete for colonies have caused wars and conflict. So trying to establish colonies can make some of the scenarios we’re trying to insure against more likely.