Let’s start with an assumption:
We want to live meaningful lives on a healthy planet and we want the generations to come to be able to do the same.
And a related definition of true sustainability (Ehrenfeld 2009):
[Sustainability is] the possibility that humans and other life will flourish on the planet forever
We are currently using 1.7 Earths each year. This means that ‘we use more ecological resources and services than nature can regenerate, through overfishing, overharvesting forests, and emitting more carbon dioxide into the atmosphere than ecosystems can absorb’ (source).
Our current way of life is deeply unsustainable.