"Didn't DC say that they would keep the best of the Trislander fleet to service Alderney? So why does everyone think he'll turn his back on the rock?"
No one thinks he will stop all flights into Alderney, the common belief is that he will stop the Southampton - Alderney route.
In the BBC quotes as linked above he acknowledges that dropping the SOU-ACI run will kill the island, but he does not say it wont happen. Even if he did, many would not believe him.