Unions can maintain their independence but we would collectively be better equipped to battle the airlines in contract negotiations by being able to prioritize out wants throughout all work groups at all airlines.
The airlines, for decades, have aimed to get what another airline was able to extract from their workers. I remember being in JFK in the early 90's and noticed from day to the next how Delta eliminated their cabin workers and made them a vendor supplied workforce. That cause other airlines to seek the same with their workgroups because of them getting to maintain the same Labor expenses.
Today, most of us compare what other airline workers get paid and want the same, plus more. The airlines do the same thing but on the opposite end of the scale.
Knowing that, we haven't modified our approach towards negotiations. Aside from that, we haven't pooled our unions to collectively stand on something and show the industry that none of the unions plan to give in on outsourcing, for example. Rather one group gets picked off, usually the weakest because their airline may be in a precarious financial state, and eventually the other groups are asked for the same.
We just argue about AMFA v TWU, Association v TWU, Association v IAM, TWU v IAM but the airlines seem to work together while we fight each other.