Sustainability is the long-term viability of a system or practice. There seems to be a belief that a good system must be sustainable. This is not necessarily true. Coal, for instance, is a resource that will eventually be exhausted at the present rates of consumption before it can be replenished by natural processes. This does not mean that industry should not use coal; however, it does mean that eventually it will be necessary to switch to a different fuel. For the time being, though, coal could very well be the most efficient fuel to use for some purposes, such as power generation.
In the long run, nothing is sustainable. An individual human life is unsustainable; it will eventually end. That does not mean that the life was not worth living or that it was undesirable that it should ever end.