Phileas is right, there is a case for an airport but a modest and diverse one. The problem with Plymouth is that it has water to the South and Dartmoor plus a low density area of Cornwall surrounding it. Counting the South Hams it takes 20 minutes longer to drive from Kingsbridge to EXT than PLH ( RAC Route Planner) so that catchment area is split. With the dockyards hardly thriving is there really a case for a major airport at PLH?
Like so many of these regional airports its future lies in diversification and perhaps very limited scheduled flights, a delicate balance. Let's buy the Ark Royal and an Islander.........!.