People from the right cultures can adapt to live in crowded areas. The Japanese have done it, utilizing cultural values which Americans have usually regarded as humorous. I doubt if your Mark I American could do it. Too used to space, and where it's crowded act out through drugs, crime, violence.

I absolutely agree that a colony supported by Earth could make it. No doubt. Transportation, as you said, is the key.

And I get the feeling that these folks are talking about self-sustaining colonies built for the long-term. I think a better idea to attempt to ensure human survival in a catastrophe would be to build domes here on Earth, supplied for the long-term.

Take some nuclear submarines and refit them for human habitation. If something hit have them head for the bottom of the sea. They could come up every so often and sample the air, look around, and go back down if they had to. (Do not allow any politicians or lawyers on board. None.)

When it was safer, they could surface in an underground pen, and transport themselves to a nearby dome. With 15 or 20 subs each harboring some critters, lots of frozen eggs and sperm, and some people mankind could have a chance.

With enough warning, some domes could be created which could house and automatically feed and water various herds of animals.

Now that, to me, is worth fantasizing about.

Arf