For all those folks worried about their quick OS promotions being stopped by BA FOs transfering over: BA management have already offered BA secondments to OS retining mainline seniority. They don't care about OS FOs promotion prospects.
The single seniority list has no detrimental effect on anyone vs. what is already on the table. It just stops mainline being shafted in the years to come (and gives OS pilots a whole host of options in later life).
There is no reason for pilot in-fighting.