The problem is that the industrial project will attract more people to the site, and a new town would be built with families living there. So even in the imagined best scenario, when you shut down this project, lots of people would lose their jobs. They would have to do some other businesses to support themselves. The light pollution won't be eliminated.
Actually, in my imagined best scenario, we would develop better means of producing energy that make a solar farm in the middle of the desert less attractive, causing working there to become less lucrative and encouraging people to find new jobs. The megaproject winds down naturally due to that changing economic landscape (like how I expect the town where I grew up in Texas to die when the oilfield workers no longer frequent it), and the dark zone is reclaimed for astronomy or whatever.
