I suspect they'd have been better off going through the BA proposal and saying what they dont like and why they dont like it. BA could then chuck some sweeteners in and start sensible negotiations re the new payment, which as you say needn't be a disaster. After all, with flexibility in MBTs and no premium trips BASSA could easily then say, well how about you give us a bidding system and then we'll think about it. Of course the new payment also means new fleet is not nearly such a threat with no premium trips to pinch.
Just coming out with uncosted polar opposites is never going to work. Whatever the final solution I suspect it will be an amended BA proposal rather than an amended BASSA one. Accept that and get negotiating!