Problem
What does "sustainable" mean to you? Try to come up with a definition and some examples that fit your view. For example, does it mean that the practice in question can be done for a long time or forever? Does it mean that we are generally living in a way that permits the continuation of humanity, society, prosperity? Does it require that the same resources and conditions of nature be maintained or increased?
To me, sustainability means that the practice you are talking about can continue for an extended period of time, potentially forever. I think it also means we are living in a way that lets the Earth thrive and maintain it's resources.