Thanks for all these thoughts. Just to make the point that Thoreau uses the word "Wildness" to mean much more than wilderness. He also means an attitude, a way of life which is not saturated in human goals and achievements. I suspect he thinks that if we lose that wildness (And its roots in nature and wilderness), we will lose our ability to understand some part of the human, and that will lessen us, the way losing the last of the Manx speakers, etc. would. It is not all or only about wilderness preservation: it is about preserving something wild.
yours, Peter T.