Put simply they are non-negotiable because BA say they are. It takes two sides to negotiate. If one side is stonewalling then you don't get a negotiation. That's how it is. Unless you want to go on strike to force a negotiation now as well!
BA numbers on the B scale will inexorably rise, (as are the numbers of pilots on BARP, and as they rise so the emphasis in negotations shifts) but the point you are either still missing or are unwilling to accept, is that without the changes there is no more BA mainline recruitment. It's the end. The choice is thin end of the wedge or thick end of the wedge. No wedge is not on offer.