FRatSTN; you provide a very comprehensive description of the various airlines that have come and gone at STN.
You mention some airlines who have not left, such as AirBerlin, GermanWings etc. I'd suggest that the airlines who can fly routes ex STN that are not head to head with FR on routes to primary airports, these will have a sporting chance of success.
STN has potential, but to what extent is FR's scale at STN a deterrent to airlines starting up services?