We keep the one that is cheapest to maintain, of course. I suppose they could mothball the ones they don't keep out in Arizona somewhere, for possible resurrection.
Keeping an expensive aircraft in the air to provide jobs is not a valid reason. Neither is keeping them around for their aesthetic value, as they don't need to fly to appreciate what they look like: we have cameras and recordings of these things in flight that are really cool to look at.
Again, drones are the future, combat aircraft are the past. Too bad drones are not as much fun to fly. But we pay a million dollars to train a pilot to fly a thirty million dollar plane for defense, not for the amusement of the pilot.Neither should we build or maintain these things as a make work project.