BA/Virgin
There have been numerous posts on here re BA/Virgin and comments attributed to their views as regards Bhx and/or other regional airports.
Hec7or, your view I think is closest to hitting the nail on the head in as much as that you have taken the discussion away from what the airlines want and concentrated more on what the passengers want.
Emirates stands a shining example of this from Bhx, as does Lufthansa and KLM. Once you strip away the significant business travel (extremely valuable as that is) the majority of that which remains has onward travel outside Europe.
These are people that have taken the conscious decision to avoid not only London, but Heathrow. As Hec7or rightly says, for anyone planning onward travel, the place is a 'hellhole'. 'BA wants to fly out of Heathrow, Emirates don't have to'
If another runway were to be added to Heathrow, then so too passenger numbers would escalate. It would still be a 'hellhole'...
Perhaps it is the publicity element that needs improving; you CAN fly to virtually anywhere in the world from Bhx. If BA and Virgin don't want Bhx, then conversely, you the passenger don't need them!