I am not suggesting one way or another. One example to add weight to my point would be the current case at the ECJ between BA and BALPA, with regard to holiday pay and where both flight and cabin crew on leave believe that they should be paid full pay (allowances etc), rather than their basic salary. If you were to go from flying, to not flying, when other employees were flying in place of you, then I imagine their may be a case to argue.
In not doing anything, then BA would be paying staff not flying and also new crew flying too. It all goes back to the dismissal/re engage SOSR defence, if BA was to act.