Why it won't happen? Raw percentages. If 50% of the workgroup is in favor of screwing over the other 50%, you'll wind up like HP & US, or UA & CO.
The percentages supporting APA are well in excess of 60%. There's no way a minority union can swing 20% support in their favor absent a huge action of malfeasance. It's something the TWU might be able to pull off, but APA is far more reasonable in comparison.