The airlines that are merging are going to be very busy just trying to keep their airplanes in the air. It is havoc. Airplanes look the same on the outside, but they are completely different inside. Maintenance programs differ. The procedures pilots use differ.
People have different computer systems.
They have different flight-dispatch systems. They have different pricing systems. They have different accounting systems. At airports, you're at opposite ends of the airport. So that means you've got to negotiate with all those other lease holders at the airports.