The Alderney - Jersey route is a little more political than it is financial.
The route was operated for many years, by various airlines and incarnations of airlines. Remember Rockhopper, as it was then?
Aurigny pulled out of the route saying, and reiterating to this day, that they would never return to it. How much Guernsey (island of not Bailiwick) government influence that decision by wanting Alderney's travellers to go via GCI, ideally over night, has influenced that position can only be guessed at.
We all know that the Bailiwick's population is shrinking, which does suggest that subsidies are needed for transport. But that is hardly new is it, the amount and accessibility of the subsidy are harder to pin down though.
As far as Alderney tourism, Jersey is a much richer marketing ground than Guernsey. Both in terms of Jersey residents, many of whom have links/fondness/desire for what Alderney has to offer, but also in terms of visitors accessing the islands via the largest island's air links.
Add to that the base of business, with numerous people in Alderney travelling to Jersey regularly and the ones who would but cannot afford to wait three hours in Guernsey.
How many seats a day this equates to is hard to define, but the demand is there for sure and in both directions.
You are quite right of course, flying in an empty aircraft from a distant base of operations is madness. I would rather charter an Aurigny aircraft, but the notional 4 Tris and 3 Dornis are yet to be made real.