Not sure that I am convinced. I don't think the situation at St. Mary's is that different. The runway may be shorter, but choices at Alderney are apparently not that much greater if even the Dornier 228 cannot make it. Why should an airline be able to charge a premium there just because it operates from a shorter runway? That does not define the willingness of the customer base to pay a certain price. What defines prices is the need to get off the island, the alternative modes of transportation, competition between airlines etc. That said, the environment at the Scillies is much more challenging as there is a competing daily ferry service on a proper ferry and a competing helicopter service.
The Skybus fare targeted at locals between St. Mary's and St. Just is 125 GBP return, whereas Aurigny between Guernsey and Alderney currently is at 88 GBP. Southampton on Aurigny is 216 GBP, Exeter on Skybus is 230-260 GBP. Not cheap, but it is not as if you could travel on Aurigny more or less for free just because of the Trislander. So will the islanders stay at home if prices go up? Maybe a few will, but because the Islander only has half the capacity, that would not make it impossible to make a service viable. If you have four Trislander flights today, you could have six Islander flights and still would have slashed capacity by a quarter.
After all, Alderney is an island. The locals may be price-conscious, but realistically, their choices are limited if they ever want to leave the island. I would agree, of course, that demand is higher in the Southwest, but three or four Islander flights a day do not seem to be impossible to fill.