A codeshare would be nice; it's the lack of a through booking that has always put me off from flying into NQY and then onto St Marys. However Skybus charge very high fares which are essentially flexible so probably makes little difference on the outbound journey- different matter on the return.
As for fixed wing, as others have said, St Marys airfield is too small for most operators. When BA ran Brymon they did explore extending the runway at St Marys but this was rejected (rightly so in my opinion) as being too invasive.
More importantly, fixed wing doesn't deal with the very wealthy clientele who use Tresco. The island is now virtually all timeshares, so a predictable number of people travelling on a regular basis. There are regularly 4 and even 5 flights a day for a large part of the year, although this drops to 2 a day in winter. My experience is that these flights are nearly always full and make a significant contribution to BIH overheads which, if lost, would make this apparently loss-making route something that would be ripe for complete closure.