For the avoidance of doubt I agree that BA wanted to dump the route. You are right, they would much prefer their MIA passengers over Heathrow.
There has always been an internal battle within BA that anything outside of Heathrow is diluting the "core" business of feeding long haul and Europe.
BA are gone where there is a market they will be replaced. People seem to be taking this personally. I seem to remember most of those code shares were ex Heathrow. Not every BA flight is a code share remember, and the BA1503 was not pushed internally within BA.
The faction that saw no future in long haul from Manchester won the day.