It seems that a corner stone of BAs argument is that there may be a future cost liability if OS pilots are on the MSL.
Surely if OS is a booming success, a lot of OS pilots will not want to join mainline as they will get earlier commands and hence better pay by staying within OS.
Alternatively, if OS fails, then the OS pilots who have passed the BA recruitment process will make up some of the 250 pilots required for mainline this year (assuming that BA agree to employing OS pilots using the same selection criteria)
The middle ground is the problem area..........but BA do not know what will happen anymore than Balpa do.
BA managers have continuously highlighted the fact that Balpa are threatening IA to protect something that may not happen. But surely BA are also guilty of not re-writing the mainline agreements to protect something that may not happen.