Sustainability is an ethical concept that things should always be better in the future than they are at present.   It is a commitment to continuous, monotonic improvement in the human condition.  That is, sustainability is not an optimization or a maximization principle.  It is about the direction, not the pace.  Thus, sustainability is about the compass, not the speedometer.

Continuous improvement in the human condition requires that all humans have the opportunity to improve their condition -- that includes rich and poor, present and future.  Thus, sustainability is not inherently egalitarian nor hierarchical, except that it does admit that the rate of improvement in some populations may be slowed so as to ensure that all segments can improve.