There are typically two types of “sustainability”.
Type one is “sustain the current system by maintaining the status quo”. This means for every person you want born, you need one to die. For every tree you cut down, you plant one. You balance things and assume that there are no “leaks” in the system.
Type two is “sustain the current system by providing for future population growth”. This means for every person you want born, you need to find the resources required for them to live the lifestyle we’ve become accustomed to. This means working on the most efficient means of extracting and utilizing resources to keep up with population growth. For those of you who have read Chung Kuo by David Wingrove, his imagined future world is a “type two sustainable”.
As you might have noticed, these two types of “sustainability” are diametrically opposed. I’ll also note that “type one sustainable” is diametrically opposed to individual freedom.