But I can see Alan's point. We can't make everyone agree with our endgame. Backcasting works for us, but what about everyone else who is NOT on the sustainability page, as you said?
To use another Daniel Quinn analogy, Christians didn't start out in the second century with a grand plan to make the Western world Christian. It was one man/one woman at a time, one meme at a time, and then we wind up with Constantine and then we wind up with the Holy Roman Empire etc etc. Peter and Paul probably wanted to see the world follow the teachings of Jesus and they did what they could to make it happen, but they didn't go around enlisting people to agree on the endgame, without first attracting them individually with a promise of the benefits that would be conferred (i.e. heaven).
The benefits of sustainability vs. the benefits of mass consumerism are being tested slowly, but we have a long way to go.
However, getting small groups together who have the common goal of sustainability and THEN backcasting with an eye on the individual group's circle of influence can, over time, help to attract and spread the word. Eventually all these little habitats get connected with a web of common understanding--the dots are connected, and there is a cultural shift.