That whole "safety risk" argument to justify the non payment of BPP is flawed anyway. A major point of the court case (IMHO) is that nobody gets assessed until only a few months prior to the actual upgrade!
They can't make a blanket statement that SOs aren't suitable for upgrade unless they've actually made an assessment of every candidates. Only if then the candidate is considered not suitable for upgrade could the BPP be not paid. And I very much doubt that many would be assessed as non suitable if that assessment took place 18 months after joining (like it used to be by the way, not so long ago).