By a sustainable future many people imagine a future like the past. A future like the past is only possible by not existing. That is what such misguided sustainable futurists hope to achieve i.e. a state of non-existence a.k.a death.
A truly sustainable future would be based on the principles that the western civilization was built on which makes existence possible today in a manner which was not believed to be possible by skeptics back then.
For instance, consider the Green Revolution; it prevented all the extreme worldwide famines predicted by the science fictions of the past.
Dr. Norman Borlaug of DuPont did not make sure that the Aussies ate everything in a serving so that people of Calcutta would automagically get the food the Aussies did not waste. Dr. Borlaug created high yielding crops and literally saved a billion lives in the third world. i.e. Dr. Borlaug saved more lives than Mother Theresa by invention and rational manipulation of reality.
Don’t hate the fact that human beings are capable of manipulating nature. That is our means of survival. The verdict you pronounce on your means of survival will determine whether you continue to exist or be wiped away by reality.