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."
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."