I can see the BA side to this... the answer is predicting and controlling costs.
If the Openskys pilots are placed on the mainline seniority list and openskys is open to mailine pilots as a bid then training costs will soar.
Lets face it most openskys first officers would be bidding straight into mainline as soon as their (presumed) 5 year freeze is up, these pilots have to be replaced and ratings / training is a big cost. Also with mainline pilots taking commands in openskys they will be there for again say 5 years then bidding back to mainline, this generates costs on both sides to mainline and openskys. By keeping openskys a seperate entity these costs can be predicted and controlled.
I cannot see a mainline pilot giving up his pension rights to go for a command in openskys either.... next Balpa will be demanding that the "mainline" pilots retain their company pension contributions whilst working for openskys.... then open skys will want the same deal etc again costs soar.
I can see the fear of the mainline pilots over the threat of openskys to their t and c. Perhaps a different approach will be needed... an agreement that openskys will never operate into UK airfields? never "W" pattern into the UK etc? and it is kept an entirely seperate entity in europe with no rights either way of pilots transferring to mainline or vice versa.