This is such a frail argument in my opinion. Have a look at Dubai for an example of what humanity is capable of when we put our effort into something. If we truly wanted to, we could colonize any area of the planet. We just have no real motive to do so. Colonizing Mars is just the next step for our species. It's going to force us to tackle some really, really big engineering and civil problems, but we're going to eventually get there because that's what we do. We explore and we create new civilizations. Always have, always will.