But it is the primary requirement of Mars colony advocates, so it cannot be dropped. It is perfectly legit to argue that their goals are useless and terrible and wasteful, if you want, but arguing "there are more habitable spots on Earth" isn't on point. Their goal is not to colonize "some difficult, hostile environment" - their goal is to colonize another planet.
The argument here is, at least in part, that achieving it on Earth would likely increase the chances of succeeding on Mars. To rule that out on the basis that one's goal is to colonize another planet (as opposed to, for example, showing that achieving it on Earth would not help with the ultimate goal) is a way to avoid considering that argument, not a refutation of it.
1) many Mars advocates want to colonize Mars to provide more resources and land to alleviate overpopulation. In that regard, it being of Earth does not matter.
2) even if it does, the point of GP's counterargument is not "if you want to colonize a hostile environment we have those here on earth" it's "your goal is extremely far outside of our current technological capabilities. We can't even colonize hostile environments in our own backyard."
