To me, "sustainability" means a situation in which your descendants are able to confront their own problems, rather than the ones you exported to them. If people a hundred years from now are soberly engaged with phenomena we have no nouns and verbs for, I think that's a victory condition.
On the other hand, if they're thumbing through 1960s Small World paperbacks and saying "thank goodness we've finally managed to pare our lives back exclusively to soybeans and bamboo," well, that's not the end of the world, but it's about as appealing as a future global takeover by the Amish.
Or, as I've always said, the reward for confronting and solving the massive problems we face today will be that our dependents get to wrestle with more interesting problems in the their own day.
Well, what with climate change, persistent toxins, radioactive wastes, depleted top-soils, species extinctions and all, we might be a little late for that. So I guess it behooves us to export some solutions along with the problems, eh?
I like how Michio Kaku, the futurist, describes the different states of civilizations in terms of their energy needs. Class 0 civilizations are still burning fossil fuels. Class I civilizations have figured out how to harness all terrestrial sources of energy: perhaps getting heat from the core of the Earth itself. Class II figures out how to leverage the full power of the solar system's energy, perhaps by building a full solar grid around the sun.
I'd like to see us make strides in my generation towards becoming a Class I civilization: to harnessing natural sources of clean energy in ways that support our existing populations. And to build the grid in a scalable enough way that future civilizations can continue to grow..