The walking scenario is still unlikely despite a shortage of 744s. You'd need to find a single customer or group of customers who would commit to taking all BAWCs capacity on a long term basis at the same or better rates. There's no way BAWC would permit GSS to bump their cargo from some flights in favour of another customers, they're far too mercenary for that. I think the moment GSS failed to put BAWC first then BAWC would be the ones threatening to walk. They can take the loss, GSS can't.
I did ask Jim McAuslan about GSS prior to a recent BALPA meeting. He was of the opinion that the long term goal is for all the cargo work to be flown in-house by BA.