Raking over the history of BA and MAN isn't going to change anything. The reality is that both moved on a long time ago, and both are doing quite nicely without each other thank you very much.
BA's primary business and profit source is feeding passengers via its LHR hub. The fact they are able to sustain eight shuttles a day from MAN means they still have a sizeable following in the north who are happy to do that. Those who prefer not to be hubbed through LHR now have a large and growing choice of other options to choose from.
BA is now part of IAG, and IAG is part of Oneworld. BA doesn't appear to have too many problems finding slots for new services from LHR when it needs to. And if one day IAG does finally run out of LHR slots, or for other reasons wants to increase it's footprint at MAN and the regions it doesn't need BA to base its aircraft there. It can do it by codesharing on its Oneworld partners, signing up new codeshares, or acquiring more airlines. IAG's recent interest in EI is perhaps a case in point.
As for the airport, we will never know if MAN would have been more successful had BA hung around for longer than it did. What MAN does now have is an excellent and growing selection of long and shorthaul point to point, point to hub, low cost and charter services - arguably the best selection of any airport in the UK.
And where would BA park those based 777s anyway - certainly not at T3 !!