Sure.
But fudpucker is suggesting that once the Trislander goes, scheduled services from ACI will disappear completely, while my point is that services may become more expensive, less frequent and on smaller aircraft, there will still be flights, i.e. not everything depends on the Trislander.
To put things in perspective, the cheapest (round-trip-based) fare from St. Mary's to St. Just is 32,50 GBP one-way on Skybus, and there are last-minute day return fares on the BIH helicopter for as little as 25 GBP one-way.