You would go bankrupt trying to operate a Dash 7 on that route.
V1 ooops: You forget the yields possible on short, high end routes to/from islands where many HNW individuals have their weekend abodes. While not a monopoly (the ferries provide service) there is a substantial demand for a premium air service to the Isles of Scilly.
(Having said that, there must have been a serious obstacle if Bill Bryce didn't try it with the Dash 7 - I bet it was the issue of fire cover for more than 20 pax!)