As part of the airport redevelopment plans, Stobart plan to build a freight distribution centre at the airport, which will become their main operating base. If the plans fall through, then Stobart have threatened to move their base outside of Carlisle, at the cost of 650 jobs.
The freight hub will provide valuable rental income for the airport, which will help to subsidise operation of the airfield. As most of the facilities at Carlisle will be newly built, they can be designed in such a way as to keep the maintenance costs as low as possible. This will give the airport a much lower cost base than the likes of Durham Tees Valley or Manston. This may be enough to keep the airfield viable.
Personally I am sceptical at the demand for passenger flights from Carlisle. I really cant see the demand for flights to London. Carlisle is well connected to London by train, so there is no advantage of flying. Unless connections can be offered at the other end, and at a reasonable cost, then there's just no demand there.
Dublin I can see working on a twice weekly service to allow for weekend breaks, but I can't see demand for a daily service. There may also be limited demand for European charters.
I think the main possibility for Carlisle exists in the freight market. With Stobarts moving their base to the airport and opening a freight distribution centre, then it offers a good opportunity to operate an airfreight operation, flying cargo in to Carlisle for distribution through the Stobart network.
The business case for Carlisle airport is certainly marginal, but at the end of the day it is Stobart that is footing the bill, so if they are prepared to pay for it - let them. We all gain from a nice new airport, that may then be able to attract some flights.