I think BA got what it wanted and needed from B|MED at the time. They also got a fairly extensive code-share agreement for a long period of time.It is easy to say BA have no interest in serving the mid-haul markets, but due to bilaterals BD is the only UK airline that can serve many of the routes.
I imagine that the domestic and Euro routes where BA already operates would be the first to have their slots reassigned, it makes logical sense to cull those first and then the mid hauls. Logically the BD fleet of A319s and short haul A320s might end up refleeting LGW as more A321s and long-haul aircraft enter the BA fleet at LHR.
I dont know if any of this would happen, but it would seem a possible scenario in the event of an IAG buy-out of bmi.