Nellis is in no real danger of shutting down. THe Air Force Fighter Weapons School is there and the huge Red Flag exercises are run there. I finished one late last year and go again in month myself. Dreamland is the huge testing area to the northwest along with the huge bombing range. Also at the base there is a huge red horse squadron and many other tenants.
Nellis and all its gunnery and other ranges is about 60-70% the size of the state of Nevada.
Most of the land is useless for anything else (apart as a wildlife sanctuary which it actually is, most military terrain has abundant wildlife that doesn't mind the occasional bomb or jetblast too much, the animals know to stay out of the dangerzone).