I think we're talking about different kinds of scale here. A habitat that could be destroyed by "a few people building a submarine" isn't a backup policy for humanity. Not here, not on Mars, not anywhere. I'm imagining a proper city-sized habitation for few million people - with industry and agriculture to support it. A truly separate colony that trades with the rest of Earth when it needs to, but can survive on its own.

Also I really don't like how the arguments so far are about preventing the starving people from coming over and messing things up. It strikes me as very "we need to keep the pleb out" kind of thinking, no different than what some rich people are doing.