Just to be absolutely clear - being on the BA MSL would not stop OS being meritocratic within itself and need not incur any additional costs or complexity because Balpa is willing to negotiate those away.
Balpa is willing to ensure there are NO costs involved in putting these guys on the MSL but BA will not negotiate
Why?
The only believable answer is because they want to use OS to under4mine Mainline Terms etc over the next decade.
BA managers still try to persuade us that their guarantees can be trusted when most of us have first hand experience of one or more BA managers not being trustworthy and BA as a whole has shown itself willing & able to ignore such previously given guarantees.