Re: A technical comment, not a historical one
I think terraforming is a red herring. I don't see it as necessary to colonization.
If I were to drop you where Las Vegas is 300 years ago in the middle of August, you wouldn't last much longer than you would on Mars.
If there's a reason to live somewhere, we'll figure something out.
Of course, I don't see an overwhelming reason right now to live on Mars except for scientific stations. Heck, how many big cities are there in Antarctica? Hopefully, this will change.
It's entirely likely that the main reason for large numbers of people to move into space hasn't really been discovered yet, or if it has, hasn't been recognized. 300 years ago would you have guessed gambling as a major incentive to colonize the Nevada high desert?