I agree with your definition of sustainability to an extent. It is indeed about preventing the over exploitation of resources. I however believe that substitution only helps alleviate the high demand on certain resources but does not help truly sustain a population. Man made solutions also come with man made problems that may hinder the ability of a certain resource to sustain itself. I feel there is a very fine line between using substitutes to support the sustainability of a resource and using it to replace the resource all together.