Cityjet has surplus capacity since cutting back at London City. Therefore in my view, as long as these routes cover their direct operating costs, it makes economic sense for Cityjet to operate them for the time being.
If that is the case Cityjet would have been a good candidate to take on the BFS and EDI flights that EZY have dropped from SEN; there's a proven market for both routes, filling A319s on occasions (just not often enough). The local newspaper and its website have been full of disappointed punters wanting the EDI flights to continue from SEN. Smaller equipment would surely generate high enough yield to be consistently profitable on these routes.
I wonder if SEN management even bothered to contact Cityjet?