I don’t see the point of removing big waaagh just for the sake of splitting BT.
it always be popular and perform in the meta , giving players the possibility of playing only gork with IJ or only Mork with Kruleboyz , or gorkamorka with big waaagh .
removing it will not improve neither IJ or Kruleboyz , just remove one playstyle .
also IJ and KB don’t need to have separated BT to have better enhancement / mount trait / battle tactic or whatever. You can improve them in a single battle tome , just add more pages to the BT .