Living sustainably is an ambiguous goal in so many ways, for one thing. Secondly and more fundamentally, there's no direct connection between achieving it by some clear definition and being able to colonize the rest of the solar system. We could just as well end up living very unsustainably on Earth while still pulling off all kinds of colonial feats elsewhere in our system. Again, nothing stops the one from being done while the other is the case.
> We could just as well end up living very unsustainably on Earth while still pulling off all kinds of colonial feats elsewhere in our system.
That is theoretically possible. It's theoretically possible billions of humans could achieve perfect cooperation and peace on one continent whilst all others remain in the usual cycles of conflict. Both these seem highly unlikely to me, given natural constraints.
