Hello EGHQ
You raise some interesting points, my friend. Speaking from my vantage point, as an experienced operator at regional airports, I would say Plymouth's best future as an airfield is to become totally GA-focused and ignore the needs of the fare-paying passenger. Commercial passenger operations add a raft of cost which is not sustainable there. A well-managed GA aerodrome, like Shoreham, can be sustainable with the right operating model and then, of course, it will attract non-scheduled business traffic as the bonus.
I think the terminal building, with some re-modelling, lends itself to becoming an 'instant office' type operation for small businesses, with a small area designated for aviation movements. Probably everything under the tower could do that job quite nicely.