If the definition of sustainability that you are using is "sustainably living" then it isn't [insert ideas associated with the sustainability movement here]. No

If the definition of sustainability that you are using is "sustainably living" then it isn't [insert ideas associated with the sustainability movement here]. Not automatically, anyway. If all we care about is continuing to live (excepting the limitations imposed by entropy) then continuously importing resources from somewhere else is a perfectly valid solution. For that matter, no amount of efficiency will ever eliminate the necessity of importing resources. Pushing that necessity into the future is probably a good idea, of course. Continuing to live is not all I care about. But mature space infrastructure would positively contribute to doing just that, even if a colony on Mars might not in and of itself.