I haven't read all of the sustainable threads and their many posts. But, I scanned through some. What I'm left with on the overall question of sustainable golf is that it is along the lines of David's observations in that golf - nor anything else requiring input of knowledge and application of practices - is in a steady state, and the concept of accepted definition of sustainable is always a moving target, as stated.
To me the most important aspect of the idea of long term sustainability is that we have a long line of tradition by the practitioners of the science of turf grass management, that reflects this sort of free and intellectual exchange of ideas, as Don is one of the best and most curious of these turf professionals, and that they have the passion to reach out to each other and other sciences to continually examine the issue in the spirit of what keeps ideas alive and the process "re-sustaining" itself to find better methods, constantly.
I think that there is always a two edged sword that both helps and hinders sustainable process. The negative is selling products for the sake of keeping the profits going for corporate and economic interests, that aren't always in the best interest of long term good use of resources to protect environment or promote most effective economical operations, leading to the long term unsustainable trajectory of golf and its courses. Yet, the same market forces can work to allow truly valid and economically effective process where everyone wants to buy and use what helps promote effective practices. So, what is successful, may by default become that which promotes sustainable process.
To separate the positive trends towards sustainable practices from those that are wasteful and harmful at the end of the road, we need professionals to be fully engaged and passionate (as Don promotes) both for their own long term self interest and sustainable living, and for the health of the game-industry-courses-economy. Sales/marketing profits must balance with free exchange of science advances and professional practioners passion to advance their knowledge and trade.