United will feed JB just enough biz to keep them in biz long enough for UAL to reestablish their foot back into JFK, then they will put the screws to JB in some future move when they tire of the competition and are ready to make a go of it on their own.

Sorta like the chickens accepting a wolf in the henhouse believing it will help defend them from the pack of wolves waiting to pounce, until the wolf gets hungry. JB's better play would have to done what it could to keep another competitor out of JFK to the best of their ability. Then if UAL wanted back in JFK bad enough, at least demand entry into star alliance for allowing them to invade your home hub.