WHBM - Its wrong to say every single full-service has been a commercial disaster.
There are success stories that have operated only until LHR/LCY slots have become available - such as Aegean, Luxair - or that operated for many years until wider financial issues within the company took hold - such as CSA (who are gone from LHR now also), Cyprus Airways.
American Airlines achieved excellent loads (apprecianting yields are the important bit) but would never have lasted as they were there only until Maxjet/Eos were killed off, and El Al would probably still be at STN now were it not for the better offer that enticed them to LTN at the 11th hour.
I'm not saying there have been no disasters, but some airlines have made a success of the place. Full-service carriers coming and going is not a phenomenon unique to STN.