BALPA represents me and hundreds like me as well as thousands of BA pilots. For some reason BALPA thinks it is better for a new operation with new routes to be flown by pilots who already have huge opportunities, rather than my bunch whose future looks distinctly dodgy.
Actually, BALPA thinks that it's better for those joining PL/OS to be on the master BA seniority list than not. This gives those who work for OS (you, maybe?) the chance to move into BA proper at a later date, should you so wish...
If I were a Jetstar pilot I might be quite happy with the situation.
Why? How does less choice improve your lot?
The basis of most of the assumptions you are making is that "BA pilots" want to "grab" as much work as possible and keep it for themselves. Fact is, we've got quite enough work on at the moment and to operate the OS flights requires recruitment into OS and/or BA == jobs for EU pilots. The choice is to join OS and that's that or have the possible opportunity to transfer to BA mainline; both choices have the same T&C to start with. Which is preferable to most intelligent people...?