The point being is that Plymouth shouldn't just be 'ticking over' MrsChips, it should be doing much better than it does.
If you look at the effect of opening up Portsmouth dockyard again and moving the work up there, it has a vibrant economy, fairly buoyant house prices and the surrounding areas in Hampshire benefit.
Plymouth has an airport that has just stopped flights to London, and from what you have said, a section of the population that can't see if the RN move out of Plymouth altogether then Plymouth is screwed, as there is nothing going in to replace it.
It's the peripheral businesses that are hit as well - the nightclubs and bars on Union Street for example. Students don't have the money that workers do, especially with higher fees coming in.
My family live there, I lived there and in a 20 mile radius until I moved abroad; I still have my house there, and go back frequently. My family are and have been involved in the Dockyard and the RN there and in the people I know in Plymouth, there is concern about the economic effects of the withdrawal of ships and submarines from Devonport. We evidently move in different circles.