Andrew,
I'm pretty sure that Brymon never put the DHC7 in to ISC, partly because the was only one DHC7 based in the South west and was normally occupied on other routes.
The Jetfoil is passenger only, no vehicles, a double decker, and could seve St. Mary's, Tresco and indeed other islands, if, literally, flies across the water at circa 50mph, carries a hell of a lot more passengers than even a DHC7, it can't be compared similar to a clapped out ferry, on the Ramsgate/Ostend the ferry would take 4 hours, the jetfoil 90 minutes!
The scenario now being faced, it seems, that islanders choice will be to be dumped at a grass airfield in the middle of nowhere as opposed to flying in to, literally, Penzance town centre.
For the tourists they either take a train to Penzance or another Cornish station, then public transport, if any, to St. Just or they need to travel down by car.
A 'flying boat' could transport then town harbour to town harbour a lot quicker than messing around with surface transport between towns/stations and aifields in the middle of nowhere!